Though MS have similar things since 2000's, but I just found about it today.
Wikipedia is quite more informative
For Linux guys, that means MS supports CC and X11, GDB, ls, kill, grep, awk, and the list goes on and on. As I understood from wikipedia, it comes with more than 350 such utilities.
CC is a wrapper around VC++. X11 doesn't come with a working X server so you'd use an open one like Cygwin/X.
To avoid confusing the reader, it's originally an implementation of the POSIX specifications (Portable Operating System Interface, an IEEE standard). I don't know why they took it from a defensive point of view and claimed it is only there to support UNIX apps..
UNIX apps can be source-ported, but not binary-ported. That means you have to recompile them on Windows to work there.
There were such a POSIX-compatibility implementation on Windows that I've heard of and used extensively (like Cygwin), but this is the first time for me to know that MS have "paid" developers to work on that :D :D
Wikipedia says that for Vista it only comes with Enterprise and Ultimate editions.