From LQWiki

GUID Partition Table (GPT) replaces the Master Boot Record (MBR) used with BIOS.

GPT is a standard for the layout of the partition table on a physical hard disk. It is a part of the Extensible Firmware Interface (EFI) and is in use today on Intel macs.

Linux support is spotty. It is not supported by WindowsXP, but Vista promises support.

see: http://en.wikipedia.org/wiki/GUID_Partition_Table

GUID: Globally Unique Identifier

see: http://en.wikipedia.org/wiki/Globally_Unique_Identifier

rEFIt see: http://refit.sourceforge.net/