Why do we need to use assemblies ?

Question

Dear experts,

 

I read of a sample project code which contains adds assembly="System.web.extension.desgin" and follows by publicKeyToken etc.

 

This is totally new to me.

 

Why do we need to do web application using assembly ?

 

And what is the publicKeyToken about ?

 

From where can we obtain this publicKeyToken ?

 

Also, is there any step by step tutorial that covers asp.net web form application based on the above scenario together with stored procedure that I can study and try out things ?