this might be a little easier to understand, my brother described it to me like this:
your control arms basically keep your axle in check while it's flexin it up. there is an upper and lower control arm on each side of both axles. The upper connects to the top of the axle and the lower connects to the bottom of the axle. they keep the axle from rolling or twisting front to back. they also guide and support the axle as it flexes and such.
stock, your jeep comes with what are known as short arms. if you looked at the side of your cherokee up near the front wheel and crouched down you'd see the lower control arm connecting to a bracket on the unibody. when you lift a jeep, the angle from that bracket to the axle gets much higher. this will make the ride rougher and it is tougher on that arm and bracket because all the bumps and vibrations are being directed up into the body instead of spread along it. plus, your axle can only go as far as that arm allows it to. with short arms, thats not too far.
a long arm lift kit replaces those short, stubby little arms with longer ones that mount farther from the axle. this lowers that angle, effectively smoothing the ride and making the distribution of pressure along the body more efficient. it also allows that axle to drop down lower when you need that extra bit of flex.
as for parts that a long arm lift comes with, it will come with all of the control arms, as well as the new brackets that attach them to the new positions on the underside of the unibody. to mount these, the old brackets will need to be removed and the new brackets mounted. some kits require the new brackets to be welded on and some are bolt on kits. for the bolt on kits, you need to drill holes in the unibody.
as for a regular kit, it would come with just replacement arms and shocks, etc.
advantages to long arms? more flex, better ride, strength, all kinds of fun stuff like that. the only disadvantage is the price. most long arm kits are pricey. also you have to modify the jeep. it's definitely worth it in my opinion though.
advantages to short arms? well... price i suppose. dont get a lift bigger than 3 or 4 inches though or you will need to get long arms.
haha that was kinda long:wierd: i hope that helped a bit though. pm me if there's anything i left out or you need some more help. and for anyone else, please correct me if i'm wrong, this is just my basic knowledge on lift kits. thanks!