MBR is the standard partitioning scheme that's been used on hard disks since the PC first came out. It supports 4 primary partitions per hard drive, and a maximum partition size of 2TB.
GPT disks are new, and are readable only by Windows Server 2003 SP1, Windows Vista (all versions), and Windows XP x64 Edition. The GPT disk itself can support a volume up to 2^64 blocks in length. (For 512-byte blocks, this is 9.44 ZB - zettabytes. 1 ZB is 1 billion terabytes). It can also support theoretically unlimited partitions.
Windows restricts these limits further to 256 TB for a single partition (NTFS limit), and 128 partitions.
Only Itanium systems running Windows Server 2003 and Windows Vista systems with an EFI BIOS can boot from a GPT disk. The other operating systems mentioned earlier can use GPT disks as data disks but not boot disks.