For batsmen, R stands for right and L for left (and HB for -handed batsman).
For bowlers, it's a little more complicated.
R usually stands for right-arm, and L for left-arm, especially for the seam bowlers (they're the ones who don't try to turn the ball with wrist/finger movements, but just attempt to get it as a high speed as possible, and also try to get the ball to land on the seam so that it gets awkward bounce).
There's different pace categories - usually four (correct in order, might be slightly off in the speeds):
F (fast) - 145+kph (90+ mph) - bowlers like Harmison, Shoaib Akhtar, Brett Lee
FM (fast-medium) - 130-145kph (85-90 mph) - bowlers like Flintoff, McGrath, Pollock
MF (medium-fast) - 120-130kph (80-85 mph) - bowlers like Pathan, Naved-ul-Hasan
M (medium) - 100-120 kph (70-80 mph) - most bowlers in the County Championship.
So RFM would be right-arm fast-medium. Grammatically, that doesn't really make sense, but never mind.
Then you've got spinners. They're classified in three main categories:
OS - off-spin. Right-arm bowlers that turn the ball from leg to off stump, away from the batsman. Usually done by means of your fingers, hence also known as fingerspin. Bowlers such as Harbhajan Singh.
LB/LS - leg-spin. Right-arm bowlers that turn the ball from off to leg stump, into the batsman. Usually done by a flick of the wrist, and may also be called wrist-spin. Bowlers such as Shane Warne.
SLA - slow left-arm. Spin action again, but this time with your left arm (so that someone using his fingers would turn it into the batsman).
Then there's also LBG - leg-break googly - when you have a leg-spinners action but can turn it the other way (away from the batsman), which is known as a googly. And there's also SLC (slow left-arm chinaman), dunno how much it is used these days, but it's a slow left-arm bowler with a leg-spinners' action.
Feel free to comment, btw, I'm only 98 % sure on the details here...