Free to step out of the aircraft, but typically you can clip yourself into the inside of the plane with a carabiner. My first time jumping out of a perfectly good plane was at 15,000 feet out of a plane with no "normal" door on the side: as we climbed to altitude, we were sitting on the floor of the plane looking towards the tail - and straight out the gaping door on the side of the plane. It's a weird thing to look (almost) directly ahead and see the ground at that angle.

Someday I'll jump out of a rear-loading plane -- as long as it's not a static line jump... since I'm not military, that probably won't be a problem though ;)