That's an extraordinary question to ask like that. If you were a complete beginner you would get a flood of replies introducing you to Google. But you seem to be in a programme for a Master's degree, so no excuses. This is on a par with someone doing a Master's in English Literature posting "can someone tell me who Shakespeare is? Has he written a lot of books?".
What research have you done on this yourself? Have you read any tutorial media on Java at all? What exactly is the thing that's confusing you after doing your research?