Imagism was the first organized movement in modern American poetry and it advocated the use of sharp, clear language to vividly depict an image, much like a painter or a sculptor. It aimed to render language like a clear glass through which a precise visual image can be conveyed without distortion. Especially, it stressed to use ‘the exact word, not the nearly-exact, nor the merely decorative word’ and to ‘render particulars exactly’. Here is a brief description, along with principles from Imagist manifesto.
The quintessential imagist poem is Ezra Pound's In a Station of the Metro. He tried to capture a moment he experienced in the underground metro station; the original poem he wrote had 30 lines, which he eventually condensed into a final poem of merely 14 words:
In a Station of the Metro
The apparition of these faces in the crowd :
Petals on a wet, black bough .
Wikipedia has a good entry on the poem, and you can read Ezra Pound's own thoughts on it here. Other imagist poets discussed were H.D. (Hilda Doolittle) and Wallace Stevens.
William Carlos Williams was influenced by imagism but he also experimented with many different styles of poetry. Contrary to traditional poets who preferred the natural over artificial, Williams insisted on seeing beauty, significance and life in the modern, made-made world. Consider Lines and Between Walls, in both the reference to green, broken glass comes up. Williams also experimented with poetry to show that anything, no matter how trivial, could be an appropriate subject of poetry. For instance, This Is Just To Say, which is actually a poem made out of a refrigerator note left to his wife, and The Red Wheelbarrow. Al Filreis compares this to Duchamp's Fountain, which turns a urinal into a work of art, and makes a similar statement about the subject matter of art. Other poems of WCW that were discussed include The rose is obsolete, which discusses how rose as a romantic symbol is dead, and Portrait of a Lady, which represents the frustrated, failed attempted to make a (linguistic) portrait of a person (especially as it was done in traditional poetry and art), and does so by a dialogue between two voices, one of a poet, sketching the portrait in a conventional way, which is more of a caricature given the terrible metaphors, and the other of the skeptical, modernist alter-ego of the poet.
"It did not seem to Plato any insult to philosophy that it should be transformed into literature, realized as drama, and beautified with style; nor any derogation to its dignity that it should apply itself, even intelligibly, to living problems of morality and the state."