I have one Sharepoint Project, in that i have three servers

Hi All,

I have one Sharepoint Project, in that i have three servers

1)Front end web server
2)Application Server
3)Back-end server

Now my dought is, what are the services in front end server and what are the services in application server.

How i find is it front end server or application server.

Any one help me out?

Thanks in advance.