P2P, is essentially Moving things through an AE network, without them actually being on the network. The most common use for this is to avoid using the "expensive" Dense cable. In all honesty though, it depends entirely on distance which works out cheaper, if you only going about 10 blocks, the dense cable is likely going to be cheaper (It also depends on how much you value diamonds and fluix dust). The P2P Tunnels work in Pairs, and these must be on the same network, for obvious reasons. You'll also need a Memory Card to store the information on.
The main thing to rember, P2P is single input, multiple output. You Shift-right click on a P2P tunnel with the memory card to save its setting to the card. This makes it the Input Tunnel. You can then right click this Memory Card on other P2P tunnels on the same network in order to link them together. (I might have got this back to front, I sometimes mix these up.)
To the Tunneled System, it's pretty much treated as if both sides of the tunnel are right next to each other. This isn't just useful for reducing cost of cabling over long distances, it can also be used to transfer Liquids, Redstone Signals, Items, or Power over the network. It's a very flexible tool.