Broadband & P2P, Shaping,

I am trying to find some simple way to explain how the following works.
Using a 512 ADSL broadband connection why you get faster speeds when connection to a web site and slower speeds when using P2P. I have tried to explain how the link is shaped and certain traffic is given priority to other. I just can?t seem to get it over however. Like how does the network know the difference and so on? Any ideas where I can find more on this in a simple diagram showing how the traffic is split up into its different bands or channels using different protocols.