应用程序池是由一个或多个url组成的一组，由一个或一组工作进程提供服务。应用程序池用于分离共享相同配置和应用程序边界的IIS工作进程集。

应用程序池用于分离共享相同配置的IIS工作进程集。 应用程序池使我们能够隔离我们的web应用程序，以获得更好的安全性、可靠性和可用性

应用程序池为应用程序提供隔离。并且提高应用程序的可用性，因为每个应用程序池都在自己的进程中运行，所以一个应用程序中的错误不会导致其他应用程序池。我们有一个共享池，在它下面运行多个web应用程序，还有一个专用池，在它上面运行单个应用程序。

An application pool is like a pond, if I create 2 application pools, first application pool has 100 fishes and another application pool has 200 fishes, here fish is like an application in application pool. They are managed by worker processes. Best advantage is: if pond number-1 has bad water and cases all fish are effected then there is security of fish in pond number-2. Like this if any application pool is affected by any problem but there is not any effect of this problem in application pool 2 so security improves, and another benefit is that is you provide all the necessary authentication and rights to all applications in a single application pool.

我是投票第二高的答案，但如果有人觉得有用的话，我想在这里添加更多的细节。

短版:

IIS runs any website you configure in a process named w3wp.exe. IIS Application pool is feature in IIS which allows each website or a part of it to run under a corresponding w3wp.exe process. So you can run 100 websites all in a single w3wp.exe or 100 different w3wp.exe. E.g. run 3 websites in same application pool(same w3wp.exe) to save memory usage. ,run 2 different websites in two different application pools so that each can run under separate user account(called application pool identity). run a website in one application pool and a subsite 'website/app' under a different application pool.

长版:

每个网站或网站的一部分，都可以在一个应用程序池下运行。您可以使用应用程序池控制网站的一些基本设置。

You would like the website to run under a different w3wp.exe process.Then create a new application pool and assign that to the website. You would like to run the website and all it's code under a different user account(e.g under Admin privileges),you can run do that by changing Application Pool Identity. You would like to run a particular application under .net framework 4.0 or 2.0. You would like to make sure the website in 32 bit mode or have a scheduled recycle of the w3wp.exe process etc.All such things are controlled from iis application pool.

基本上，应用程序池是一种通过进程边界在web服务器中创建区域的方法，并将url集路由到每个这些区域。更多信息请访问:http://technet.microsoft.com/en-us/library/cc735247(WS.10).aspx