Application pool is created the each and every website.
Application domain is created to one domain purpose.

IIS process is w3wp;

Every application pool in IIS use it's own process; 
AppPool1 uses process 3784, AppPool2 uses process 5044
Different applications in Asp.net will use different 
AppDomain; 

AppTest2 and AppTest2 are in different AppDomain, but in 
the same process.
 
What's the point to use them?
 
Application pool and AppDomain , both of them can provide 
isolations, but use different approches. Application pool 
use the process to isolate the applications which works 
without .NET.  But AppDomain is another isolation methods 
provided by .NET.
If your server host thousands of web sites, you wont use 
thousands of the application pool to isolate the web sites, 
just becuase, too many processes running will kill the os. 
However, sometime you need application pool. One of the 
advantages for application pool is that you can config the 
identity for application pool. Also you have more flexible 
options to recyle the application pool. At least right now, 
IIS didnt provide explicit options to recyle the appdomain.