Concurrency - tasks run in overlapping time periods. Not necessarily at the same instant. For example, multitasking (time-slicing) on a single-core machine.
Parallelism - tasks literally run at the same time, e.g., on a multicore processor.
Process vs Thread
A process is an instance of a program running on a computer. A thread is a dispatchable unit of work within a process.
Threads (of the same process) run in a shared memory space, while processes run in separate memory spaces. A thread is a light-weight process, but a process is a heavy-weight process. A process is maintained by operating system whereas a thread is maintained by a programmer. On a multiprocessor system, multiple processes can be executed in parallel. On a uni-processor system, though true parallelism is not achieved, a process scheduling algorithm is applied and the processor is scheduled to execute each process one at a time yielding an illusion of concurrency.
