Most miners talking about KHash/s are talking about Litecoin mining, while those talking about MHash/s are talking about Bitcoin mining.
The hash algorithm for LTC (scrypt) is about 1,000 times slower than the algorithm for BTC (SHA-256), which explains the speed difference.
There is no average rate. Different people mine a different speeds.
BTC: https://en.bitcoin.it/wiki/Mining_hardware_comparison
LTC: https://github.com/litecoin-project/litecoin/wiki/Mining-hardware-comparison
A Gh/s is anything up to 10,000 hashes per second
and a Mh/s is anything up to 100,000 hashes per second.
No, not really.
1 K = 1,000
1 M = 1,000 K = 1,000,000
1 G = 1,000 M = 1,000,000,000