A thread pool is a group of threads initially created that waits for jobs and executes them. The idea is to have the threads always existing, so that we won't have to pay overhead time for creating them every time. They are appropriate when we know there's a stream of jobs to process, even though there could be some time when there are no jobs.
Here's a nice diagram from Wikipedia:
与恶龙缠斗过久,自身亦成为恶龙；凝视深渊过久,深渊将回以凝视…
