Custard: Smooth, thick cream made with egg yolks and milk. Melts fast.
(Also known as soft serve)
Ice Cream: Malleable, utilitarian, long lasting flavor. Frozen, sometimes with items mixed in. Can be a pain to scoop.
("Traditional," found in Ben & Jerry's and Cold Stone Creamery)
Sorbet: Typically fruity and sweet, frozen water that occasionally has ingredients other than flavoring mixed in. Seriously, it's just frozen water.
(Typically offered as an alternative to Ice Cream in ice cream shops)
Frozen Yogurt: Yogurt, except it's frozen.