Pigpen is trivial to read if you know the key, and easy to decipher if you don't.
The Pigpen is so called because the key looks like an aerial view of lots of pigs in pens!
Sometimes the key letters are filled in using a different order, however as the cipher is essentially monoalphabetic, this presents no real decoding problems
There are many examples of this kind of cipher being used, one of the more famous is in Conan Doyles' Sherlock Holmes story, the Case of the Dancing Men. Though the symbols are different, the idea of replacing letters with symbols is the same.
The cipher simply replaces each letter with a symbol according to the key.
The above reads 'This is an example of the pigpen cipher'