you'll probably get better answers than mine but they are missions created by members where you fly with and against AI drones. Ranger, in particular, puts some awesome missions together but most of the guys that put them up semi regularly really take the time to make them good and debugged. I've found they help me a lot with my formation gunnery, ACM, and SA. Additionally just like the offline mode it doesn't take long once you've been sent back to the tower to start and get back into action (even though you have to start over). The drones can be tough to beat at first.
My typical night on AH for now is fly offline and shoot down all the planes flying the circle using slashing attacks and so on. Anything but from six oclock. Then run a couple of offline missions, then off to the TA for some instruction if someone is available or just get into a mix up with whoever is available.
Try them I think you'll like them. Oh and lastly after you've slammed your stick into the wall once or a dozen times don't forget to come back here and thank the guys for their efforts. <S>