Doubt regarding SAP WebAs

hi frds,

1) what is the use of WebAs?

2) All SAP Components can run in SAP WebAs?

3) SAP System means SAP WebAS ?

4) XI Components like IR,IDand SLD..etc reside on SAP WebAS...like this other SAP Netweaver Components like EP Components also reside in 2 stacks of SAP WebAS?

5)How the XI Message flows from sender Business system to Receiver Business system via WebAS ? plz give example?

6) What is Difference between SAP System and SAP WebAS and 3rd Party Systems?