I was always confusing those. Trying to remember them now.

1TiB is Tebibyte = 2⁴⁰ bytes ~= 1,099,511,627,776 bytes (more then 1 trillion)

1TB is Terabyte = 10¹² bytes == exactly 1 trillion bytes (1,000,000,000,000 = 4 groups of zeroes)