Commons are the workhorse cards of any set — the ones that make up the bulk of the checklist. They're the least rare and therefore the least valuable cards you'll pull. A typical common? The base card of a non-rookie player who isn't exactly lighting up the headlines.
Commons sit at the opposite end of the spectrum from chase cards and hits. But don't sleep on them entirely — if you're a set builder or a completist, commons are essential pieces of the puzzle. And every once in a while, a player on a common card breaks out, and suddenly that card isn't so "common" anymore.