Bitcoin, Dogecoin and Peercoin all use SHA-256 Cryptography
Litecoin uses Scrypt.
They all differ in price, total coin amount and transaction time.
Doge actually uses scrypt, and I don't really know how peercoin works but they claimed that they are hybrid.
bitcoin = first cryptocurrency
litecoin = first scryptcoin and altcoin
doge = based on a joke
peercoin = ?
Yeah peercoin uses a proof-of-stake/proof-of-work hybrid system (the first cryptocurrency to use POS).
The network transaction fees are also destroyed (not paid to miners), which offsets the inflation caused by minting new coins.