In short, using their own tools, you can create, play and put up for general consumption your own mission arc. Create your own set of custom characters/custom enemies to play with/fight alongside, arrange ambushes, patroling groups of enemies, hunt for things, etc. It's scarily wide open and so much possible to do it's staggeringly impressive. Admitally, you're only able to use their already coded custom parts, and pre-formed maps, but there's so many of them that you won't notice. There's even a very nice option to import your own current character's outfit into it.
So naturally enough, I'm going with what I know : ordinary looking girls horribly proficent with medieval weaponry. "Miss Winters" is a dab hand with double swords, "Miss Helane" looks her battleaxe, and the redhaired (with streaks of black!) spellcaster "Mrs Kennedy" tends towards fireballs. [/shifty-eyes] Why, I'm sure I don't know what you mean. :P
Fittingly, everytime I test the mission, they keep killing me.