what are render elements passes used for

[haneet22]

i

was reading this wonderful tutorial

http://area.autodesk.com/index.php/t..._lighting/P10/

i havent understood the compositing part
that is how reflection or specular passes helpful?
and how to use them in photoshop?
do i save it as separate jpegs? because when i render passes they each render as different images

what about dirt map

Can anyone throw more light on this

Thanks