The "iambos" refers to the Greek word which means "skip" (going from memory of Greek Theatre lectures here). A short/weak syllable followed by a long one: . -
So an iambic pentameter is five of these rhythmic pairs (penta-meter) , "in a skipping fashion". As in Shakespeare: . - . - . - . - . -
Technically the reverse meter - long-short - is trochaic, but hey we're not so fussy, we'll still call it iambic. As long as it skips. :)
PS. Sorry johar, if you wish to cut-and-paste could you please add that you are quoting from answers.com? Ta.