What is the difference between concurrency and parallelism?

Question

What is the difference between concurrency and parallelism?

Examples are appreciated.

Answer 1

Parallelism: Having multiple threads do similar task which are independent of each other in terms of data and resource that they require to do so. Eg: Google crawler can spawn thousands of threads and each thread can do it's task independently.

Concurrency: Concurrency comes into picture when you have shared data, shared resource among the threads. In a transactional system this means you have to synchronize the critical section of the code using some techniques like Locks, semaphores, etc.