To concentrate is to direct your mental powers or your efforts towards a particular activity, subject or problem.
Memory is the ability to remember information, experiences and people.
There are some specific skills that can be learned to enhance both concentration and memory. Practicing these skills is likely to improve one's success as a student. Once something is stored in our brain, we never forget it. We may, however, have difficulty recalling the information. It is also possible that the information we are trying to recall was never stored.