A foot is a unit of metre, consisting of a combination of stressed and unstressed syllables. If stressed syllables are marked "/" and unstressed "u", the main types can be shown thus:
Iamb: [ u / ], such as "delight". (The adjective is "iambic".)
Trochee: [ / u ], such as "badger" (Trochaic)
Anapest, or anapaest: [ u u / ], such as "unaware" (Anapestic / anapaestic)
Dactyl: [ / u u ], such as "multiple" (Dactylic)
and, more rarely:
Spondee: [ / / ], such as "tooth-ache"
Pyrrhic: [ u u ], such as "such as" was until it was put in quotation marks.
It is important to remember that feet and words need not coincide. The feet in John Heath-Stubbs' line, "A caterpillar among those mulberry leaves", from 'The Mulberry Tree' appear thus:
| a CAT | er PILL | ar a MONG | those MUL | berry LEAVES |
| u / | u / | u u / | u / | u / |
That one word "caterpillar" is scattered across three feet in this five-foot line - the first two are iambs, then after a single anapaest there are two further iambs (or one iamb and one more anapaest, depending on whether you say mul-ber-ry or mul-bree). Also note that, although there is an anapaest in the centre of this line, this is still a predominantly iambic line (especially as it is within a predominantly iambic poem) - varying the feet like this can keep a line from getting metrically dull.
The process of working out where the stresses fall is called scanning, or scansion. It's easiest to do it on poems where the rhythms are pronounced; on the other hand, it can be near-impossible, or simply unhelpful, to scan free verse. The poems suggested below have strongly accented feet, and the links to metre and form go into more detail on how poets use feet.