POSIX Threads vs. Win32 Threads

I just dipped my toes into the POSIX pond and tried out POSIX threads for the first time. Until now, I'd been under the impression that there's a big architectural difference between POSIX threads and Win32 threads, but from the (admittedly little) that I tried, I didn't really see any difference.

I'm still curious though -- what are the differences (if any) between POSIX threads and Win32 threads? Are they different fundamentally, or do they just have minor differences?