GPT: GUID Partition Table

A good tool to use: gdisk

Traditional partition table stored in MBR has some limitationss: maximum 16 partitions and, the most important, a maximum single partition size limitation of 2TB.

To use GPT, there are some requirements:

1. BIOS - must support EFI: Intel-based PCs should support EFI now.
2. bootloader - must be GPT-aware: patched grub should be fine
3. OS - must also be GPT-aware

I've tried an upgrade from MBR to GPT on Ubuntu 9.04 successfully with gdisk. However, you need to reinstall GPT-awared grub to make it boot correctly!

For the detail, please read the referenced URL.