Hexadecimal, called hex for short, is simply base 16. We use base 10 in our normal counting. Hence, in hex you'd count 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, A, B, C, D, E, F, 10, 11...19, 1A, 1B, 1C, 1D, 1E, 1F, 20, etc.
The reason that it's used is because 16 is a power of 2, 2^4, and hence, it lets us represt binary data better, without the insane amount of 1s and 0s. 100000000000000000000000000000000 in binary is 100000000 in hex, greatly reducing the amount of characters that you need to type.
There are several ways to convert to hex, but I prefer PCalc from
http://www.analogx.com. If you don't want to download something, the standard Windows calculator will also do it in scientific mode.