Litecoin uses scrypt, a memory-intensive hashing function, whereas BtC uses SHA, which is merely computationally expensive. It was specifically designed to be

Litecoin uses scrypt, a memory-intensive hashing function, whereas BtC uses SHA, which is merely computationally expensive. It was specifically designed to be difficult to mine with ASICs, so as to lessen the advantages of professional ASIC mining farms and give relative power back to all the guys who built GPU mining rigs back when that was cutting-edge. The unfortunate side-effect of this is that LtC botnets are far more viable relative to the total network hashrate than BtC botnets, as the difference in power between commodity computers and ASICs as far as hashing is concerned is far less for LtC than BtC.