You complete a set of show jumps, usually 6 to 10, then straight onto timed xc/natural fences usually in same arena. Big arena's outside might include jumping water, banks etc.
Indoor arenas need careful course walking as jumps can get like a forest.
As said above very careful course walking required and those who win, especially on ponies, are super quick! It's great fun and a brilliant way to keep 'eventing' over winter.