In terms of poetry, there are two kinds or forms, based upon the "structure or pattern of organization" that a poet adopts when writing his verse.
These are called "open" or "closed" forms. When looking at a poem's form, you can observe the following; with more than one of these in a poem, there is probably a set pattern. Look for the rhyme used: it may be end rhyme (where a word at the end of one line rhymes with the word at the end of another line). There may be a rhyme scheme (which is a specific pattern of rhyme, such as ABAB, where each letter represents a sound, and the pattern is followed in a stanza or an entire poem). The meter is the poem's beat (which is found in sonnets, where, for example, iambic pentameter is often used: ten syllables in a line, with emphasis on the second syllable). There may even be stanzas used (which are often groups of four lines, but not always). There are other elements as well: these are only a few examples.
When a poem has a closed form, the poet has adopted a pattern that the poem will follow in more than one area, such as those mentioned above. As an example, a Shakespearean sonnet is a fourteen-line poem. It has three quatrains (which are four-line stanzas), it ends with a rhyming couplet (a pair of lines that rhyme with each other), it follows a specific pattern of rhyme (ABAB CDCD EFEF GG), and is written in iambic pentameter. In composing this kind of sonnet, the poet follows these parameters. Other examples of a closed form poem are the traditional haiku, the tanka, the limerick, the cinquain, and the villanelle.
Note the haiku below. It is about nature; it has three lines; and, the syllabic pattern (number of syllables) per line is 5-7-5; (note that this is the traditional Japanese format of the haiku):
“The Rose” by Donna Brock
The red blossom bends (5)
and drips its dew to the ground. (7)
Like a tear it falls (5)
In contrast, the open form poem does not follow set guidelines. There is no required rhyme scheme, rhyming pattern, or set number of lines in a stanza. One stanza, for instance may have four lines, as may the second, but a third stanza may have five lines. A concrete poem is one that is spaced out so that it creates a picture. As an example, a religious concrete poem might be shaped like an altar. However, for Halloween, a concrete poem might be written in the shape of a pumpkin or a bat. This may be the only guidelines present, and it is considered a poem with an open form.
Note the lack of form (or the "open" form) of the following poem:
“American History” by Michael S. Harper
Those four black girls blown up
in that Alabama church
remind me of five hundred
middle passage blacks,
in a net, under water
in a Charleston harbor
so redcoats wouldn't find them.
Can't find what you can't see