Prosody - a theory of poetry composition or an organizing principle - within the bounds of which one can build the sructure of the poem. The poet may use any aspect of language on which to base a prosody, but most English language prosodies have to do with counting syllables (aka meter).

Meter - ("numbers") means "measure," and when one measures a line by counting syllables in that line, one is writing VERSE, not prose. If one is counting simply the number of syllables in a line and measuring out a certain number of them per line, then one is using syllabic prosody, and one is writing syllabic verse. If one is counting in each line only those syllables that, for some reason, are more HEA-vi-ly EM-pha-sized (stressed) than others, then one is using accentual prosody and one is writing in accentual verse.

If one is counting not only all the syllables in theline, but all the stressed syllables as well, and arranging them in an alternating pattern of some kind - in series of "verse feet" - then one is using accentual-syllabic prosody. If one is not ocunting syllables at all, but is arranging lines in grammatical parallels or phrasal units, then one is not wriring verse, but prose, which many people erroneously refer to as free verse.