report is dispalying in application serve as per normal 
requirement.it is used in transaction code -
se38.techniaclly speaking report is an excuatable program 
with 3 stages.
DATA INPUT (select screen)
DATA PROCESSING (select statement)
DATA OUTPUT (defining output types in 
write,uline,skip,output fields)


module pool program  are dialog program
dialog program are used to call transaction
this are used in customizing of screens
this are more flexible than reports

Reports are created from abap editor
Report is evaluate data from database,and displayed in list.
Report are standalone programs.
Reports are controlled be events.

Module pool is nothing but transaction, created from object
browser.
Transaction is the sequence of screens. In module pool at
least one screen is participated.
Module pool is not standalone programs.
PBO,PAI and screen flow are important events