Concurrency and parallelism are two related concepts which deal with executing tasks "simultaneously". From Sun's Multithreaded Programming Guide:Concurrency: A condition that exists when at least two threads are making progress. A more generalized form of parallelism that can include time-slicing as a form of virtual parallelism.Parallelism: A condition that arises when at least two threads are executing simultaneously.These two definitions are too formal to be understood easily. On the Internet, there is one image which demonstrates the difference between these two in an easy way:Much e...