L -keeps it in the lowest gear, usually used when towing heavy load slowly and you don't want to upshift when you release the gas pedal.
I seldom use it since the tranny uses the 1st gear when you're just accelerating in D mode.

2- keeps it in second gear- useful for relatively long steep downhills for engine braking but it is mostly used on icy roads where you don't want too much torque which will cause wheelspin.

D- standard driving mode uses the 1st-to the highest gear. I use this 99% of the time.

O/D off- keeps it off the overdrive gear so the tranny uses only 1st to 3rd gear. useful when engine braking on a light long downhill, but seldome used nowadays

Most people try use 2 or L when they need acceleration, in my opinion you don't need to do this, keeping it in D will do, it will shift down if you need more power anyway, if you really need to put up the revs and power during overtaking, release the gas a little bit then stomp it to the floor, it will make the a/t go into the emergency power mode, it will downshift and will keep it in that gear till near redline or when you trottle down a little bit.

R- to go into reverse (duh!)

P- park mode