example.
The modifications are intended to improve or refine the Bitcoin protocol. Litecoin has a quicker transaction confirm time, scrypt as proof-of-work and higher number of coins. Primecoin finds prime numbers at the same time it confirms its blocks. Namecoin has a special key-value storage.
In theory, an altcoin can be started from an existing Bitcoin blockchain, although no known example exists at this writing. (See also Fork