Process File: csrss or csrss.exe
Process Name: Client/Server Runtime Server Subsystem
Description: Windows client server run-time subsystem handles Windows and graphics functions for all subsystems.
Company: Microsoft Corp
System Process: Yes
Security Risk ( Virus/Trojan/Worm/Adware/Spyware ): No
Common Errors: N/A
Copied from a website, the above is what it is, what exactly it does I don't know but since its Microsoft created I'd leave it until someone tells you more about it.