Expansion packs.
CIV4 would just be the bare game, the complete edition has all the expansion packs included. Honestly unless you're just crazy over Civilization I'd just get the 20 dollar one. The game isn't great enough to honestly merit paying for expansion packs... More then likely you'll be bored with the game within a couple of weeks.
However, if you are a fan, or don't care about wasting money, might as well get the full thing to experience the game in all (albeit minimal) glory.
