A diff gives differential action all the time. One tire can spin faster or slower than the other, but both generally have power. The tightness is adjustable on a ball diff, which means you can adjust how much diff action occurs.
A one-way is very different On Power vs. Off Power. On power. it locks up, meaning both front wheels always have 100% power. Off power, each wheel goes into "freewheeling." The oneway pully is slowed by the motor, but the outdrives basically disconnect, and can go their own speed, each one idependently. It's very much like a pan car, or offroad truck, or any other rearwheel drive car, at this point. What this does, is while on power, it's locked up, which gives much more positive traction. There's no loss due to slipping. It makes it more effeciant. Off power, the freewheeling action gives lots more steering. The down side, is that you lose all front braking. This is why they're usually reccomended for larger flowing tracks only, because you don't need the brakes.
A center one-way, gives something from both worlds. You keep the front diff, but lose the front braking. On Power, it's just like a normal car, with both diffs. Off Power, the entire front belt/diff/wheels freewheel from the centershaft, as a single unit. (If that makes any sense) It gives more steering than a direct drive layshaft, but not to the extent of a front one-way.
When to use them: Pretty much, if you need more steering, and don't use the brakes, throw a one-way in it. Either run the center, front, or both. The first time you use one, it might feel kinda weird, but once you get the chassis working with it, and learn how to drive it, you can knock tenths and full seconds off your lap times in a hurry.
I hope this helps out, let me know if something's unclear. I'm no expert, but I think I've got a good basic understanding of these.