Being that your palm is generally facing towards you when you bring food to your mouth, that'd be a chin-up
A negative is the part of any lift where the angle between the joints involved are increasing. What that really means is that it's the part of any lift where you aren't actively moving a load. It is when you are returning the load to the starting position. In the case of a pull-up or chin-up, the negative would be when you are lowering yourself back down.
So in order to do a negative pull-up, you would generally stand on a stool or chair or a bench and start the rep with the bar already at your chin. Then, you simply lower yourself down to the floor very slowly. That's one rep. Now you get back on the chair or whatever and reset for the next rep. They're extremely effective at building strength.
Assisted pullups are -- unlike a negative -- a full rep, up and down, but they are done on a machine that can be found at most, if not all, commercial gyms. They utilize a counterweight system which subtracts from your body weight whatever weight on the rack you put the pin in. So if you weigh 200 lbs and you put the pin on 75 lbs., then if you do a pull-up, you will effectively be lifting 125 lbs, not your full body weight.
A lat pulldown is another pulley-based machine which doesn't require you to lift your own weight in any manner. You just pull the weight down to yourself while keeping a straight back. Simple enough.