Anyone help me out with understanding this;

http://www.dsl-warehouse.co.uk/product.asp?pr=WAG54G

Is it a modem aswell as a router? Cos on a net search it shows up as a modem aswell as a router, but i don't know if thats right, as that page says you plug it into an ADSL microfilter? WTF is one of those?

If I buy one of them, can I plug it straight into the phone socket and connect PC's to it and it will work? Or do I need a seperate modem?