The language of poetry is concrete; that is, it relies primarily on words that refer to things that can be recognized by the senses rather than on abstractions.  This fact guarantees that poetry will be constantly creating images.  The term "imagery," however, usually refers to a special kind of image-making, the use of metaphor and simile.  Simile is a comparison that uses the terms "like" or "as"--"the winter was like a wolf," for instance.  Metaphor states that one thing is the other--"the winter was a wolf."  The general term, "metaphor" is often used to cover both metaphor and simile.  It is common for creative writing teachers to say that metaphor is superior to simile because it indicates a more intense degree of identification.  How effective a comparison is in context, however, is more important than the grammatical construction the writer uses, and many of the most famous metaphors in the language are actually similes. 
     Imagery is one of the most noticeable features of all poetry, and the reader, therefore, is likely to ask what it is for.  In fact, people have been asking that question for the last two thousand or so years.  Typically, they have received the simple and common-sense answer that imagery exists to explain that which is unfamiliar by comparing it to something more recognizable.  This is a very convenient answer and, like most convenient answers, is not true.  One can easily test this explanation by picking out the comparisons in a random selection of poems.  If you do so, you will find that sometimes the strange is compared to the familiar, sometimes the familiar to the strange, sometimes strange to strange, and sometimes familiar to familiar.
     The fact is that imagery, like language itself, is simply a fact of poetry, one that can be turned to any use the poet finds for it.  The human mind is forever arranging things in categories.  Words, for instance, are nouns, verbs, adjectives, and so on.  In biology things are classified by genus, order, and species.  This kind of organizing activity does not exist only on the logical level, but in the subconscious as well.  There, things logically very different are grouped together in symbolic patterns which consciously we may be hardly aware of, but which are more basic to our reaction to life and the world than any of our logical categories of science, social science, or grammar.  Such patterns are often revealed in myth and dream.  Poetry is like dream in that it often uses this largely unconscious symbolic language.      
     When most of us read Robert Burns' "My Luve's like a red, red rose," we accept the image without question, even though we may object to other more logical images.  In most respects women and red roses have very little in common.  The fact that we do strongly connect the them, however, is shown by the multitude of poetic images connecting them and, at a more every day level, but the custom of giving roses to women.  The anonymous beauty of Waller's "Go Lovely Rose," makes the connection.  The more famous Virgin Mary is also a rose, in spite of the fact that there is an air of sexuality about the whole business of rose giving.  We can understand this image best, though, in William Blake's "The Sick Rose":

                                      O Rose, thou art sick.
                                      The invisible worm
                                      That flies in the night
                                      In the howling storm

                                      Has found out thy bed
                                      Of crimson joy,
                                      And his dark secret love
                                      Does thy life destroy.            
     With the added imagery of beds and worms and passion, we might begin to see here that the rose is a vaginal image, and apparently a universal and timeless one as well.  This is a subject in itself, however, and I have a full essay on subject of the vaginal significance of roses which I might some day add as a page.  For now, we will move on to other images.
     But if the origin of imagery lies in the symbolism of the unconscious, much of a poet's imagery is fully conscious, and used for logical and easily indentifiable purposes.  For instance, in Romeo and Juliet, when Juliet says,

                                     My bounty is as boundless as the sea,
                                     My love as deep . . .

                                                                          she is using the sea as a comparison to her love because love cannot be measured or quantified, because we have to use comparisons to talk about love at all.  If we say that a well is deep, we are making a literal, factual statement; if we say that a love is deep, however, we are automatically making a comparison, because "high," "deep," or "wide" are terms that can be used literally only in relation to space.
    In the passage below, Alexander Pope is speaking about critics whose names are preserved forever in the works of great writers, not because they themselves are great, but because they have discovered some small mistake in punctuation, spelling, or grammar:

                             Even such small critics some regard may claim,
                             Preserved in Milton's or in Shakespeare's name.  
                             Pretty!  in amber to observe the forms
                             Of hairs, or straw, or dirt, or grubs, or worms!
                             The things, we know, are neither rich nor rare,
                             But wonder how the devil they got there.

Such critics, Pope says, are like insects accidentally preserved in amber--common, ugly, and unintersting in themselves, but interesting because of where they are.
     The poet is here doing two things:  he is showing us how absurd it is that such people's names appear in conjunction with those of great men like Milton and Shakespeare, and he is suggesting that these critics are like grubs or dirt.  In short, he does not like such critics, and is trying to make us dislike them too, that he holds them in contempt and wants us to do so as well.  The simile here is being used as a put-down.
     Probably the most famous simile of twentieth century poetry is T.S.Eliot's:

                      When the evening is spread out against the sky
                      Like a patient etherized upon a table.

This comparison does not create a very vivid picture of the sky, and in any case, we have seen far more evening skies than we have etherized patients.  What is the point?  For one thing, Eliot is telling us with these opening lines that this is a very different sort of poetry than readers at the time were accustomed to.  This is not an inevitable image like "my luve's like a red, red rose," but an image we are apt to dispute at first, an image that requires us to think.  For another thing, this image establishes a major theme of the whole poem, that of being suspended between--between life and death, consciousness and unconsciousnes, action and inaction.  Further, there is a real logic to the comparison, for we think of day in terms of the sky above us, but evening in terms of the horizon, so that evening is indeed spread out, lying on the earth like a patient somewhere between the light of consciousness and the dark of oblivion.  Finally, the image reveals with startling clarity the peculiar, perverse state of mind of the speaker.
     Elizabethan sonnet writers were fond of "her eyes are like the sun."  This image, of course, means that the woman referred to has very bright eyes, but obviously this is not a very accurate comparison.  Here, the image is used as a compliment, not as an explanation.  After reading this stale and rather silly comparison a number of times, it is a relief to read Shakespeare's satiric Sonnet 130 beginning, "My mistress' eyes are nothing like the sun."  The whole sonnet in fact is a satiric comment on flattering, but trite images used by poets to praise female beauty.
     For a very different use of imagery, consider Coleridge's lines from  Kubla Khan:

                               A savage place!  as holy and enchanted
                               As e'er beneath a waning moon was haunted
                               By woman wailing for her demon lover.

If you are used to seeing women wailing beneath waning moons for demon lovers, then, according to the traditional view, you should know just what sort of place the poet is talking about.  Actually, the image here is intended to create a sense of strangeness rather than of familiarity. 
     As we have seen, imagery can serve a number of purposes in a poem, and most poems contain more than one image.  However, some poems take a single image and explore its implications at length, sometimes taking up the entire poem.  At the turn of the twentieth century a school of poets called "Imagists" made the image the central element of poetry.  Take a look at Ezra Pound's two line poem, "A Station at the Metro."  Homer has a use of images so distinct that one particular kind of image is called "Homeric simile."  These are long similes that usually appear in battle scenes.  In these some event, like a slain hero falling is compared over a number of lines to some more peaceful scene of rural life like farmers felling a tree.  Probably one purpose of such extended images is to slow the action, and to provide the hearer a little relief and breathing room in the midst of all the scenes of spinal marrow flying from severed necks, and smoking entrails rolled out on the ground.
     A slightly different sort of imagery is the use of "kennings," a kind of image central to Old Norse poetry, and to a slightly lesser extent to all old Germanic alliterative poetry (Beowulf, for instance).  Kennings are essentially metaphors, and consist of replacing a noun with a phrase that challenge's the hearers mental agility, knowledge of lore, or both, sometimes functioning almost as a riddle.  Some kennings are conventional and common, some unique to the particular poem.  The ocean, for instance, can be called "the whale's road."  "Freya's tears," means "amber," because by tradition amber was created by Frey's tears for her lost love.  "The shield of stems," would be the leaves.  "Beli's bane" would be antlers, because the god, Frey killed the giant Beli with an antler.  "Stallions of the sea" would be ships.  Kennings may have originally become popular because alliteration can be difficult in a language with a fairly limited vocabulary.  However, they became a central element in Old Norse poetry, especially in the late Middle Ages.
     This has been a relatively brief discussion of a very large subject, but perhaps provides a little food for thought.  Every age has its own values in relation to imagery.  The 20th century placed a great deal of stress on originality (Whether the 21st will do the same remains to be seen).  Therefore, one needs to be careful in using conventional images unless they are used in a strikingly new way.  Avoid such phrases as "happy as a lark," or "busy as a bee," or even "my love is like a rose."   ---Jack Hart