Kilohash per second: 1,000 hashes per second

Megahash per second: 1 million hashes per second (typically a single GPU or CPU).

Gigahash per second: 1 billion hashes per second (mining pool or cluster of GPUs).

Terahash per second: 1 trillion hashes per second (typically a single ASIC or large mining pool).

Petahash per second: 1 quadrillion hashes per second (large mining pool).

Exahash per second: 1 quintillion hashes per second (typically a huge mining pool or entire network).

Zettahash per second: 1 sextillion hashes per second (soon).