a cable modem is what you would get from the cable company selling you your bandwidth (Like road runner or equivilent). You see, broadband does not get transmitted through a phone wire - it is transmitted (most commonly) through your TV Cable line. What you need for your laptop is a PCMCIA ethernet card. These are availiable on ebay relatively cheap (like 10-20 dollars). You could probably find one at bestbuy or circuit city for 20-30 dollars. Before you get one of these, you should call up your local cable provider to make sure they offer Cable Modem service. Time Warner w/ road runner is just about everywhere except if you live in the boondocks. If the cable modem company doesn't give you a cable modem, then they should be able to help you get one from a store - they'd know which one you need. The cable modem would then hook up to your PCMCIA ethernet card and the ethernet card would go into a PCMCIA slot (all laptops have them).
Hope this was helpful