The Main difference between stored procedure and Function 
is SP's can return the value or not, but Functions should 
return the value.

And We can use the function within the Queries, but sp's we 
won't able to use it in the queries.

We can implicitely execute the sp's. But function's we 
can't.

stored procedure : It is Pre Compiled Code(PCode) it is stored  
in Database. we can reuse without compile.

Function: It need compile whenever it will run.