If you want to know it in a really simple explanation (for CRC), CRC uses a process of repeated polynomial division using a special CRC polynomial (the actual process of coming up with a CRC polynomial could fill a page of text so I'm not going into that here). Basically you take the given number, divide it by the CRC polynomial, then divide the remainder by the CRC polynomial and continue until the remainder is 0. If you want to know any more than that, get a good math book
