The boys over at Mod Central Unreal have whipped up an interview with Epic's Jack Porter which talks about "mutators". What's a mutator? Good question, it had me stumped:

Mutators are mini-mods. They are simple code classes which can be used to add and remove items, from the map, change item or level properties. They work with any gametype, so you can apply your mutator to DM, CTF, Domination, Assault, Last Man Standing. This allows mod-makers to develop simple and fun gameplay changes without having to create a new gametype.