Encyption happens by replacing each letter of the alfabet with an other letter. The keys can be showed with this circle. Here is a shift of 8 chars used.

Here will the A replaced by T, B by U, C by V etc. So will next string encrypted:

Original	Encrypted
HELLO WORLD	AXEEH PHKEW

Decription happens by the same cicle showed in the encryption example. Example:

Encrypted	Original
AXEEH PHKEW	HELLO WORLD

Ceasar ciphers are easy to hack. If you know that an encrypted A is equal to H and a P is equal to I, everything with the same encryption key could be encrypted.