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What is the use of a Thread pool? Is there a good real world example?
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The thread pool is primarily used to reduce the number of application threads and provide management of the worker threads. Applications can queue work items, associate work with waitable handles, automatically queue based on a timer, and bind with I/O.
By default, the MaxThreads of the ThreadPool is very high. Usually you'll never get there, your app will crash first. So when all threads are busy the new tasks are queued and slowly, at most 1 per 500 ms, the TP will allocate new threads.
In Java, thread pool is implemented in ThreadPoolExecutor class. There are four core concepts in ThreadPoolExecutor. A work queue is used as a “buffer” to hold submitted tasks in a thread pool. When the size of a work queue reaches its capacity, we cannot submit new tasks to a thread pool anymore.
The process of testing a condition repeatedly till it becomes true is known as polling. Polling is usually implemented with the help of loops to check whether a particular condition is true or not. If it is true, a certain action is taken. This wastes many CPU cycles and makes the implementation inefficient.
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: 
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Thread Pools from the Java Tutorials has a good overview:
Using worker threads minimizes the overhead due to thread creation. Thread objects use a significant amount of memory, and in a large-scale application, allocating and deallocating many thread objects creates a significant memory management overhead.
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