The file Win32k.sys is the kernel-mode portion (device driver) of the Win32
subsystem. In other words, it's a critical core operating system file. A
good resource to get more details of what it does is documented in the book
"Inside Microsoft Windows 2000", Third Edition by David A. Solomon and Mark
E. Russinovich. To paraphrase, it manages windows (windows manager) displays
and interacts with the graphic device drivers (Graphical Device Interafce).
I hope this helps.