If i remember, Mhash is for SHA and Khash for scrypt mining.
Mh/s is Mega hashes per second or how many million hashes per second. Kh/s is Kilo hashes per second, or how many thousand hashes per second. What xDeathwing means is that typically with GPUs, you see MH/s for SHA-256 mining and Kh/s for scrypt.