It is the simple shift monoalphabetic classical cipher where each letter is replaced by a letter 3 position (actual Caesar cipher) ahead using the circular alphabetic ordering i.e. letter after Z is A. So when we encode HELLO WORLD, the cipher text becomes KHOORZRUOG.
A Caesar cipher is one of the simplest and most widely known encryption techniques. The names comes from Julius Caesar, who, according to Suetonius, used it with a shift of three to protect messages of military significance