The structure of a poem can refer to several kinds of "configuration." First, the structure often refers to the physical composition of a poem. The structure of a haiku (a three-line poem that does not rhyme) for example, is very different than an epic (or very long) poem (which is often written in rhyming four-line stanzas). The structure may refer also to the meter or rhythm of the poem. Each line may have a specific number of syllables, and in many cases, the stress or emphasis will rest on every other syllable.
For example, William Bulter Yeats' poem, "The Ballad of Father Gilligan" has lines with a rhythm that moves back and forth because the first line has four stressed syllables (out of a total of eight), and the next line has three (out of a total of six). This format is repeated to provide a sense of a lilting or swaying walk. Note that the bolded words or parts of words are where the stress or emphasis should rest as you read:
'I have no rest, nor joy, nor peace,
For peo-ple die and die';
And af-ter cried he, 'God for-give!
My bo-dy spake, not I!'
Shakespeare often writes with five stressed syllables (out of a total of ten total syllables per line) with the stress on every other syllable. For example:
When, in dis-grace with for-tune and men's eyes,
I all a-lone be-weep my out-cast state...
Structure can be seen, then, in the length of each line which with poetry such as Shakespeare's Sonnet 29 immediately above, is constructed—to achieve a certain number of beats per line, among other things. Structure along these lines also dictates whether a line will end with punctuation (as a completed thought) called "end-stopped lines," or continue on to the next line, known as "run-on" or "enjambed" lines, as is also the case with the lines in Sonnet 29.
Structure is clearly found with the use of stanzas, which are similar to paragraphs in prose writing. The stanza usually consists of four lines, and there is often rhyme included. The portion of Yeats' poem above is an example of this.
All of the structural considerations fall into the category of "form," and include other elements such as speed, arrangement, line breaks, etc. Structure in a poem is something the author uses to put his ideas together. Four-line rhyming stanzas are rather traditional, but there are also poems written in free verse and blank verse; with these forms of poetry, structure is not based on specific rules or form.
The structure of a poem is the way the ideas are ordered. Some poems follow a strict structure such as sonnets and haiku. Some poems do not follow a set structure and are often referred to as free verse.
If a poem has a group of lines (like a poetry paragraph) we call this a stanza.
There are many different kinds of structures for poems and poets select the best structure for their ideas. We call this matching form to content (ideas to structure).