What and how much is a terabyte?

Answer 1

It is a thousand gigabytes. That is a thousand billion so it equals one trillion bytes (10^12).

Answer 2

Minor change/addition to what Rich said:
a tera is a kilo giga or a kilo kilo kilo kilo
common usage for kilo is 10^3 or 1000
(making tera = 10^12 or 1,000,000,000,000)
computer usage for kilo is 2^10 or 1024
(making tera = 2^40 or 1,099,511,627,776)

The difference is small (comparatively: ~10% at 1 tera is >90 meg difference) but sometimes causes confusion. For example, hard drives are often sold spec'd at 'common' size (to make them sound bigger), but your computer reports in 'computer' size. So a drive advertised as a 1 terabyte drive will only show up as 931.3 gigabytes on the computer's report (instead of 1024 as might be expected).