will be via your telephone line, rather than being a cable connection?
That being the case, you'll receive a broadband modem and some microfilters. The microfilters plug into your existing telephone sockets, and you'll need one for each extension socket. A telephone that's plugged directly into an ordinary phone socket on and ADSL line will not be able to make/receive calls properly - there will be loud line noise.
If you don't get enough microfilters from your brodaband provider you can buy more at any good computer shop for a few pounds each.
The microfilter splits the phone line, and has two sockets on it. You plug the broadband modem into one socket (it's clearly marked) and the phone into the other socket. You can then make phone calls on the line at the same time as being connected to the internet.
The other end of the modem cord plugs into your computer, which must obvioulsy be near enough to the phone line for the lead to reach. You can use computers on other phone extensions, but you'll obviously have to move the modem from room to room.
An alternative is to buy a wireless ADSL modem/router combination, and a wireless network adapter for your laptop; then you could connect the modem/router to your PC in any room, and use the laptop anywhere in the house, without any wires to worry about.