Regarding poetry in general, what is the structure of a poem?

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:

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'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:

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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.