1. It's a device that enables you to connect more than one computer to the same ADSL connection.
2. It connects to your ADSL modem, and distributes the broadband service to multiple computers.
3. It helps because many routers now have an ADSL modem built in. This means one less device for a start. All routers connect automatically to your broadband provider when they are booted, but many can be programmed to disconnect at certain times - at night, or when you are away.
4. Costs vary, but as a rough guide allow around £150 for a good router with built-in modem.
5.Recommendations? Opinions will varyu of course, but my personal choice would be the Nokia IP55 router with modem built in. It's a tad larger than most models but has everything you'll need, including a WAN port, so you can connect it directly to the ADSL socket on your wall. Nokia have also thoughtfully provided a serial port, so if the ADSL connection goes down at any point you can connect an analogue modem and share that connection. Cost - about £140