What exactly is IIS (Internet Information Services)

I've read the following about IIS:

The servers configured with application role support web servers trough IIS Services.

I also know that

IIS hosts internal and external websites and also web services that communicate through HTTP.

So I'm trying to figure out what exactly IIS is : is it a server (an application server)? Is it a collection of web sites and services? If that's the case, where is it stored?