A terabyte is 1 trillion bytes. A terabyte is big enough to hold about 250 standard definition movies. A related unit, the tebibyte, is 240 bytes, about 10% larger.
A gigabyte is 1 billion bytes. A typical standard definition movie is about 4 gigabytes in size. A related unit, the gibibyte, is 230 or 1,073,741,824 bytes.