Meter is the organization of speech rhythms (verbal stresses) into regular patterns, in terms of both the arrangement of stresses and their frequency of repetition per line of verse.
Poetry is organized by the division of each line of verse into "feet," metric units which each consist of a particular arrangement of strong and weak stresses. The most common metric unit is the iambic foot, in which an unstressed syllable is followed by a stressed one (as in the words "revrse" and "compse").
Here is a table of meters and their names.
Meter is also determined by the number of feet in a line. A line with five feet is called pentameter; thus, a line of five iambs is known as "iambic pentameter" (the most common metrical form in English poetry).
The most common line lengths are:
trimeter: three feet
tetrameter: four feet
pentameter: five feet
hexameter: six feet (an "Alexandrine" when iambic)
heptameter: seven feet (a "fourteener" when iambic)
Naturally, there is a degree of variation from line to line, as a rigid adherence to the meter results in unnatural or monotonous language. A skillful poet manipulates breaks in the prevailing rhythm of a poem for particular effects. John Donne, for example, rarely held to the meter of his lines for more than a few feet at a time. These lines from his Holy Sonnet 14 (1633) are written in iambic pentameter, but the stress patterns vary a great deal:
That I may rise and stand, o'erthrow me, and bend
Your force to break, blow, burn and make me new.