From the
freedictionary.com, a metrical foot is a group of 2 or 3
syllables forming the basic unit of poetic rhythm.
Metrical foots can be differentiated into different categories that
contains different patterns of stressed and unstressed syllables, for
example:
Iamb means a metrical foot that contains 2 syllables, one unstressed
followed by one stressed.
Trochee is a metrical foot that contains 2 syllables, one stresesd
followed by one unstressed.
Spondee is a metrical foot that contains 2 syllables, which are all
stressed.
Pyrrhic is a metrical foot that contains 2 syllables which are both
unstressed.
Anapest is a metrical foot that contains 3 syllables, two unstressed
followed by one stressed.
Dactyl is a metrical foot that contains 3 syllables, one stressed
followed by two unstressed.
Also, a meter is a measure of syllables, in particular, the number
of foots a line has. Thus, if we want to describe a line which has a
particular metrical foot pattern and want to express the number of
foots in the line all in one word, we can use 'numbercount'-meter.
For example, a penta-meter means 5 meters, or 5 foots. Similarily, a
decimeter means 10 foots, and a octometer means 8. We use the term
'monometer' to represent one meter.
With the first two combined, we can easily describe the use of
metrical foot in poetry. For example, an imabic pentameter would be 5
groups of foots that follows the pattern of unstressed + stressed; an
pyrrhic trimeter would be 3 groups of foots that are all unstressed
(all 6 syllables are unstressed).
Examples:
Dactylic dimeter:
/ - - / - -
"Half a league, half a league"
http://www.types-of-poetry.org.uk/16-dactyl-poetry-type.htm
Trochaic tetrameter:
/ - / - / - / (-)
"In the forests of the night"
"The Tyger," by William Blake
(Note that the last foot is incomplete)