The ISP is the business that connects you to the internet. Some of the more commonly known ISPs are AT&T, Comcast, RTC and Northland Communications.
Many of these are companies that originally were telephone or cable companies, since they were already in the process of having the major communications connected to them and then sending signals to individual homes or businesses. Most often, we still see the ISP running a physical wire (or fiber optic) cable to our homes. We then connect to their network with either a router or modem (modems when using dial-up)
You can think of the ISP as being similar to your gas station. For us to get gas to run our cars, we go to the gas station to get it. However, the gas station is the delivery point to us of the fuel from the that was first drilled, transported, refined and piped and trucked to the gas station. The gas is out there before the gas station, but not in a method to deliver to us until at a station.
Likewise, there are major cables run all over the U.S. and the world that are carrying internet signals, but to get them to our homes or businesses so we can connect our computers to the internet signal, we require an ISP.
In some places, you are limited to one ISP, and in many places now more than one ISP runs wiring to your location (or a satellite signal or wireless).
Thanks to Clay for the question.
Send your questions to Dwight Watt at firstname.lastname@example.org. He teaches at a technical college in northwest Georgia and does consulting work for businesses and individuals. His website is www.dwightwatt.com.