Here you can see an example of atomic weight, atomic number, and mass number.

The atomic number is the smaller number associated with the element and is equal to the number of protons in the nucleus. 

Sometimes, the mass number and atomic weight are referred to two different numbers.  For the reason that they serve to us, though, we refer to them as the same thing, since the difference between them is so minute.  For example, the Carbon atom below has an atomic weight of 12.011 and a mass number of 12.  This gives them a difference of 0.011, which is not very much at all.  Commonly referred to as atomic mass, it equals the number of protons plus the number of neutrons in the nucleus, as shown below. (Widau)

http://www.cofc.edu/poolel/atomic_symbol.jpg