The second basic component of the definition is “iambic pentameter,” which many people think sounds difficult even though they can almost effortlessly recognize it with just a tiny bit of instruction. But let me explain it a bit, anyway; indulge me, and let your browser help you look up any terms that seem unfamiliar.
A verse has iambic rhythm, one of the four common poetic rhythms used in describing English verse, when the syllables can be divided into pairs. Each pair (called a “foot”) has one syllable that receives more emphasis than the other, whether by loudness, quantity (how long it takes to say it), or weight (how complex the sound is). This is the “stressed” syllable. What makes the foot distinctively iambic is that the second syllable, not the first, is stressed. A two-syllable foot with the first foot stressed is trochaic, not iambic. Because both trochees and iambs have two syllables with one stress, a verse with iambic rhythm may well have trochees in it, as well; these occasional substitutions are used by writers to vary the rhythm from line to line.
Pentameter is quite easy to understand from the name: each line has five stressed syllables. Iambic pentameter, then, has five iambic feet per line. Tetrameter and trimeter are also common descriptions of lines in English verse, and even dimeter is not unheard-of. More complex rhythms often have lines of varying length (for example, “fourteener” rhythm has a tetrameter line followed by a trimeter line in each verse and two such verses in each stanza). In the sonnet, however, which gains most of its complexity from rhyme scheme and theme, the only typical variation would be the Alexandrine, now rarely used. An Alexandrine is the addition of two extra syllables (also called a “hypermetric foot”) to one line in a verse, strictly to add variety or emphasis. Alexandrines are common in long poems written in heroic verse, but in sonnets they are almost never used except in the last line of the poem.