MBR stands for Master Boot Record and was the default partition table format before hard drives were larger than 2 TB. The maximum hard drive size of MBR is 2 TB. As such, if you have a 3 TB hard drive and you use MBR, only 2 TB of your 3 TB hard drive will be accessible.
To remedy this, the GPT format was introduced. GPT is an acronym within an acronym. The G stands for GUID (Globally Unique Identifier), and the P and T stand for Partition Table. The maximum hard drive size of GPT is 9400000000 TB, with sector sizes of 512 (the standard size for most hard drives at this time).
If you have a hard drive that you would like to use and it is 2 TB or smaller, select MBR when you initialize the hard drive for the first time.
If you have a hard drive that you would like to use but not boot from and it is larger than 2 TB, select GPT (GUID).
If you have a hard drive that you would like to boot from and it is larger than 2 TB, you can select GPT (GUID), but you will also need to be running a supported operating system and the system's firmware must be UEFI, not BIOS.
If you need to switch from MBR to GPT, or GPT to MBR, you need to back up your data ahead of time to avoid losing all of the data on the hard drive when you format it.
For more information on this subject and Windows, refer to the following Microsoft knowledge base article: http://support.microsoft.com/kb/2581408.