What is an IIS application pool?

Question

Application pools allow you to isolate your applications from one another, even if they are running on the same server. This way, if there is an error in one app, it won't take down other applications.

Additionally, applications pools allow you to separate different apps which require different levels of security.

Here's a good resource: IIS and ASP.NET: The Application Pool

I second the top voted answer, but feel like adding little more details here if anyone finds it useful.

short version:

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.

Longer version:

Every website or a part of the website,you can run under an application pool.You can control some basic settings of the website using an application pool.

1. 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.
2. 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.
3. You would like to run a particular application under .net framework 4.0 or 2.0.
4. 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.

Basically, an application pool is a way to create compartments in a web server through process boundaries, and route sets of URLs to each of these compartments. See more info here: http://technet.microsoft.com/en-us/library/cc735247(WS.10).aspx

An application pool is a group of one or more URLs that are served by a worker process or set of worker processes. Any Web directory or virtual directory can be assigned to an application pool.

Every application within an application pool shares the same worker process.

Assume scenario where swimmers swim in swimming pool in the areas reserved for them.what happens if swimmers swim other than the areas reserved for them,the whole thing would become mess.similarly iis uses application pools to seperate one process from another.

IIS-Internet information Service is a web server used to host one or more web application . Lets take any example here say Microsoft is maintaining web server and we are running our website abc.com (news content based)on this IIS. Since, Microsoft is a big shot company it might take or also ready to host another website say xyz.com(ecommerce based).

Now web server is hosting i.e providing memory to run both websites on its single web server.Thus , here application pools come into picture . abc.com has its own rules, business logic , data etc and same applies to xyz.com.

IIS provides two application pools (path) to run two websites in their own world (data) smoothly in a single webserver without affecting each ones matter (security, scalability).This is application pool in IIS.

So you can have any number of application pool depending upon on servers capacity

An application pool is a group of urls served by worker processors or set of worker processors.

There can exist any number of application pools.

In IIS it is possible to create more than one application pool.

An application in different application pool runs in different worker processors.

Advantage: If an error occurred in one application pool will not effect the applications running in another application pool.