Lots of things are multi-sprite animations, but other things like swinging a weapon/tool just rotate a single sprite.
Terraria is hardly a shining example of game development and the fact it "shipped" and made a lot of money shouldn't really detract from that.
Thanks guys. But what would a good, efficient way of animating sprites be? I'm really curious,in case I want to dabble in 2D sprite-style games.