They are the same thing. The difference is in how a person views it. Some people view the glass as half empty, as missing something. These people tend to have a more negative outlook on life and a negative mindset, always finding the wrong in a situation.
Others see the glass as half full, they don't focus on the missing, but rather focus on what they have. They tend to be more positive minded, always seeing the good and assuming the best of a situation.
So its really just a matter of interpretation. Are you more pessimistic (half empty) or optimistic (half full)?