GUID Partition Table (GPT) was introduced with Intel's Extensible Firmware Interface (EFI). Nowadays, it is also called Unified Extensible Firmware Interface (UEFI) as it has been adopted by most vendors. EFI and UEFI can be used interchangably.
Unlike MBR GPT defines only one kind of partition, no more logical partitions. And there can be an arbitrary number of them. Each partition is assigned a 16 Byte ID called a Globally Unique ID (GUID).
GPT is backwards compatible with MBR based systems. It does this by faking an MBR block at the beginning of the partition table. This fake MBR makes MBR based systems think that the disk is actually one large 2TB partition.