For disk storage, 1 Terabyte (TB) is 1,000 Gigabytes (GB); for processor or virtual storage, 1 Terabyte is 1,024 Gigabytes. This difference is because the term "Megabyte," when referring to virtual storage, is widely computed in powers of 2 rather than powers of 10.
This system is based on the IBM Dictionary of computing method, which is the simplest description of disk storage. A simple way to remember the system is that 1,000 bytes is equal to 1 Kilobyte (KB), 1,000 Kilobytes is equal to 1 Megabyte (MB), 1,000 Megabytes is equal to 1 Gigabyte, and 1,000 Gigabytes is 1 Terabyte. Therefore, 1 Gigabyte is one billion Bytes and 1 Terabyte is one trillion Bytes.