Are you looking for a college course type info on this or a simple grandma computer user answer?
ISP buys huge pipe to connect to the backbone of the internet, they sell a chunk of that pipe to you, and connect your house/boat/rocking chair to them with some type of cableling, wireless signal. That needs a modem at your house to talk to their signal. You connect your computer/whatever to the modem and that talks to the rest of the internet through the ISP hardware. For more devices you can get a Router that creates a small network in your house to connect several networkable devices to the one modem the ISP gives you.