Warbands are the creatures you use in the minis game (which the Miniatures Handbook was fittingly enough written to tie in with). It's sort of like your deck in Magic and other CCGs. Each miniature has an associated stat card that lists how it acts, what its stats are (including cost), and so forth. IIRC, normal warbands are made form 200 points, while Epic warbands are 500 or more. You'll have to check the individual cards to see what they are, but you can probably find them on Wizards's website. You can find the current version
here, but those are for the new version of the minis game which is based off the 4th edition rules.