Poetry forms
Unlike prose, poetry is a structured form of communication. There are different poetical forms each with its own set of rules of how the words are to be put together.
Those rules can cover several different areas:
- Rhyme structure
- Rhythm structure
- Content
- Syntactic structure
- Any combination of these (almost), or none
Rhyme structure
Here the rules dictate which lines should end in words with a similar sound, this is usually denoted by a pattern like this a-b-a-b-c-c which would mean lines 1 & 3 should rhyme, 2 & 4 should rhyme and 5 & 6 should rhyme. Sonnets have a very clearly defined rhyme structure (3 of them) as do limericks.
Rhyme is part of the syntactic structure but is so important it gets a section all of its own.
Rhythm structure
In this case the rhythmic structure is defined. For example, a common verse rhythm in English is the Iambic Pentameter: da-Dum da-Dum da-Dum da-Dum da-Dum. This is an unstressed syllable followed by a stressed syllable, five times. Each "da-Dum" is an "iambic foot" and there are five of them, hence the penta-meter.
Blank verse is written with this rhythm while Free form has no structure. You can, of course, invent your own rhythm structure.
Content (Semantic structure)
Certain forms define what they can be about, japanese haiku should be about nature, sonnets are generally about love and so on. However this will always be combined with a rhyme or rhythm structure otherwise it would just be prose on a given subject.
Of course, poems don't have to have any actual meaning, just check out many of the poems of Spike Milligan. Or you could invent a semantic structure, such as requiring that alternate stanzas define the meaning of the thirteenth word of the preceding stanza -- or something that makes more sense.
Syntactic structure
This defines such things as the number of words or even the type of words that can be used. For example, an Etheree requires each line to contain one more syllable than the last. Japanese poems usually limit numbers of syllables.