Thank goodness, someone knows the difference!
m = milli (as in millimetres)
M = Mega
b = bits
B = Bytes
G = Giga
k = kilo (SMALL "k") although there is a body of opinion which considers k = 1000 and K = 1024
Hz = Hertz (Capital H, small z)
I've seen people quoting 512mgs of RAM which actually means 512 milligrammes!