A terabyte is a collection of about 1 trillion bytes, which is around 1,000 gigabytes.
In the metric system, tera is a prefix for 1 trillion, or 1012. So in some cases, a terabyte is defined as 1 billion, or 1,000,000,000,000 bytes. Storage media companies usually use this definition of gigabyte when selling hard disks, SSDs, and flash drives.
But because computers work in binary, computer scientists usually define a terabyte as 1,099,511,627,776 (10244 or 240) bytes. This works out to 1,024 gigabytes.
The abbreviation for terabytes is TB in both the metric system (1,000,000,000,000 bytes) and the binary system (1,099,511,627,776 bytes).
Some groups have proposed the name tebibyte for a unit of 1,099,511,627,776 bytes to prevent confusion. The abbreviation for tebibyte is TiB.