What is this virtual desktop on Macs?

[Rick]

I was in class today and a girl had an ibook with WinXP loaded as a virtual desktop. Not that I even care about Macs but I'm just curious about how that works. She said problem is that it makes her laptop slow as s***. In her words.

Rick

[LongHorn]

I remember having something at school, where Macs can run and/or look like XP.