Report means you are displaying the records on the list, by 
fetching the records from the table and updating some table
but in module pool programs you are showing the data in a 
screen and you can add different functionality like 
pushbutton, GUI menu, and PF etc. and it is called as 
module b'coz on screen you can't use subroutine ou have to 
write the code in module, endmodule

reports are driven bye events where as module pool is 
driven by transaction codes .

In Reports are Executable Program
In Module pools are execute via T-Codes

some more differences are:

1) Selection screen is only usre in reports where in module 
pool normaal , sub  and dialog screen is used.

2) in reports we display data with write statement.
   In dynpro no write statement is used we have to use 
screen components like input field , custom control, table 
control