Charms are a type of magic spell concerned with enchanting an object to behave in a way that isn't normal for that object. For example, the Summoning Charm brings an object to the caster. Charms is ... something of a catch-all for spells that aren't Transfiguration (spells that change the inherent nature of an object). In some sense, if a spell isn't Transfiguration, it's probably a Charm.