ROT13 is a special case of Caesar cipher, with a 13 shift. Only letters are changed, and white-space and special characters are left as they are.
What is interesting is that ROT13 is a reciprocal cipher : applying ROT13 twice will give you the initial input. Indeed, 2 * 13 = 26, the number of letters in the alphabet.
As ROT13 doesn't have a key as input parameter it is often seen more as a encoding algorithm or, more specifically, an obfuscation algorithm rather than a cipher.
ROT13 just makes it hard to read messages directly and is therefore often used for offensive messages or puns of jokes. It doesn't provide any computational security.