JeffB
Legend
In World of Warcraft, at least, a "dungeon" is a small-ish group activity meant for five players. It usually has 3-5 bosses, some "trash" in between them, and possibly some goals to fulfill in order to progress. When a dungeon is current content, you'd expect to spend maybe 30 minutes on it.
A "raid", on the other hand, is a far larger undertaking. You need a group of 10-25 players, who should ideally make sure to fill more specific niches than "tank", "healer", and "damage-dealer" (e.g. "This boss uses a Curse-type debuff, so you need one or two players with the ability to remove those curses"). The trash is harder. You have more bosses, often 10-15, and they are much harder, and often require a lot of coordination in order to survive specific abilities. A raid is something you do over the course of a few evenings.
Thank you for the explanation.. [MENTION=205]TwoSix[/MENTION] comment makes sense to me now.