Because most traditional and much modern poetry is written in an intentional meter, it is important to understand what meter refers to. Meter simply means the rhythmic quality of a poem. In English, which is an accentual language, the rhythm is created by the pattern of accented and unaccented syllables -- the parts of words on which we place emphasis or not.
In prosody, which is the analysis of the rhythm of poetry, "poetic feet" refer to the patterns of stressed and unstressed syllables in a poem. One poetic foot consists of either two or three syllables organized by location of stressed and unstressed syllables. The foot is the smallest unit of metrical measurement in poetry. A "foot" is the unit of stressed or unstressed syllables within a line, which is made up of two or more feet.
In English, there are four major kinds of poetic feet. If you read the words below aloud, you can hear where the stressed and unstressed syllables are:
Iambic: again (two syllables, unstressed syllable and stressed syllable)
Anapestic: intervene (three syllables, unaccented/unaccented/accented)
Trochaic: under (two syllables, accented/unaccented)
Dactylic: happily (three syllables, accented/unaccented/unaccented)
The most common meter in English poetry is iambic.
Although spondaic and pyrrhic feet are often listed as feet in English poetry, there is some controversy about this. The spondee is a foot of two accented syllables; sometimes the words "hum drum" and "amen" are listed as examples. The pyrrhic meter is a foot of two unaccented syllables. Some writers argue that these feet can be present in English prosody, though only as a substitution for another kind of foot. However, other authors say these feet are not possible in English because English constantly varies accented and accented syllables and that they are merely misidentified dactyls and anapests.