What is the difference between DDR3 PC3-10600 and PC3-10666

DDR3 stands for Double Data Rate type three but does not imply a specific frequency or bus rate. The PC3-xxxx designates the particular RAM's speed rating and means that the memory certified as able to run at that bus speed. So DDR3 RAM PC3-1333 runs at 1333 MHz while the PC3-1066 runs at 1066 MHz.

Apple doesn't provide any information about their installed RAM other than it's DDR2 or DDR3 with a specified bus speed rating. Presumably the CAS, Column Address Strobe - a measure of latency, is not needed. I assume that means the memory Apple uses will have the same overall specs as what you would purchase in a third-party market.

I find that it doesn't make any difference where you buy the RAM as long as the vendor is reliable, has a reasonable returns policy, and provides a product with a manufacturer's Lifetime Warranty.

The bus speed. MBPs one and a half years ago all came with standard 1066mhz. Now they are at 1333 mhz

Here is a good explenation of what bus speed is:
http://computer.howstuffworks.com/motherboard4.htm

the difference refers to the bus speed

And, that means they are identical. You can use anybody's memory if it meets the specs for your machine. If you're in the US try OWC or DMEM.