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 = ?
all 3 are innovation expect dogecoin, that was created just for fun.