What Are Threads?

All programmers are familiar with writing sequential programs. You've probably written a program that displays "Hello World!", or sorts a list of names, or computes a list of prime numbers. These are sequential programs: each has a beginning, an end, a sequence, and at any given time during the runtime of the program there is a single point of execution.
A thread is similar to the sequential programs described above: a single thread also has a beginning, an end, a sequence, and at any given time during the runtime of the thread there is a single point of execution. However, a thread itself is not a program--it cannot run on its own--but runs within a program.
Definition: A thread is a single sequential flow of control within a program.

There is nothing new in the concept of a single thread. The real hoopla surrounding threads is not about a single sequential thread, but rather about the use of multiple threads in a single program all running at the same time and performing different tasks.
The HotJava browser is an example of a multithreaded application; within the HotJava browser you can scroll a page while it's downloading an applet or image, play animation and sound concurrently, print a page in the background while you download a new page, or watch three sorting algorithms race to the finish. You are used to life operating in a concurrent fashion...so why not your browser?

Threads of Control