The Dink Network

A thread is a process inside a process (which is where a computer program runs, together with its variables and state) allowing you to perform several tasks in one program concurrently; you'd place every task in a thread and you can make use of concurrency in your program like concurrency for processes (aka having several processes/programs running, like a webbrowser, a media player, ...). So multithreading is using more than one thread in a program.

Take note that concurrently in a system with one processor means "letting the processor switch between the processes/threads quickly and often", in multiprocessor systems it could be really concurrent.