OEM = original equipment manufacturer (or white box).
See linkage: OEM Definition
It pretty much means that it's a retai CPU, without the retail box, heatsink, or sometimes, a full warranty.
There's nothing wrong with OEM, as long as you have the other necessary items to use the OEM part. Some, like sound cards, might not come with cables, and just a driver disk. Graphic cards are usually packaged with just the driver disk.
It's sometimes a cheaper alternative than buying retail, but those days seem to be disappearing a bit. Before an OEM CPU was about $20-50+ cheaper, now, it's more around $5-18 cheaper. Look at newegg's pricing:
Intel's 805 PD costs $93 for retail, and 89 for OEM. AMD's 4600+ retail is $210 and the OEM is $199.
So, OEM use to be an upgraders choice, but nowadays, it seems like the difference isn't worth as much as it use to.