Google Desktop is an application from Google, which indexes the contents of your hard drive (including applications, most files, PDFs, as well as web histories from Safari, Firefox, and Camino) and makes it searchable a la Spotlight.
PC users have had the opportunity to use this application for a while, and it is nice to see that Google hasn't forgotten about the Mac user out there.
Google Desktop for the Mac is a Universal application, requires OS X 10.4 or later, and is free.
So why do you need Google Desktop if I already have Spotlight? Tuaw.com has done a short review that shows you the advantages and disadvantages of Google in comparison to Spotlight.