The Second Coming
The Second Coming" is written in blank verse, which means that has a consistent meter but no rhyme scheme. With 22 lines divided into two stanzas, it does not appear to follow a particular formal tradition. However, notice that the second stanza has fourteen lines, making it the same length as a sonnet. At eight lines, the first stanza could be thought of as a fragment of a sonnet that is "interrupted" by the full sonnet of the second stanza. However, these aren’t "true" sonnets in the classic sense because they don’t rhyme.
The meter is roughly iambic pentameter, the most common type in all English-language verse. For example, iambic pentameter was the preferred meter in all of Shakespeare’s plays. It has five two-syllable "iambs" in each line, each of which approximates the rhythm of a heart-beat (ba-dum, ba-dum, etc.). In formal language, this iambic rhythm is described as an unstressed syllable followed by a stressed one. Each line, then, has around ten syllables.
Yeats’s use of this meter is not as regular as Shakespeare’s. We know this right off the bat because the very first syllable has a stress on it: "Turn-ing." Also, some lines have well over ten syllables, such as line thirteen, which has thirteen syllables (and knowing Yeats, we shouldn’t assume this is a coincidence). However, most of the lines in the poem do have around ten syllables. As far as form and meter in Yeats’s other poetry goes, "The Second Coming" is fairly typical, although he was no slouch when it came to throwing down some rhymes.