An ISP is an Internet Service Provider.
We get out Internet connections through ISPs. They provide a way for us to connect our devices to the Internet. They may be a telephone company, wireless provider, cable TV provider or just provide Internet connectivity
We originally started out using dial up service through telephones, but other companies also provided dial up connectivity. The biggest of these others was AOL, which still does exist and is an ISP and provides email services also as a part of Verizon. Some people still must use dialup for variety of reasons.
DSL was and is provided by telephone companies. Most of your Cable TV companies also provide Internet service as they can use the same cabling as they use for TV service. They offer much faster service and depends on whether they have moved to fiber cabling.
Wireless providers can also provide Internet connectivity whether for cell phone or PCs. A new and growing are in this is proving high speed wireless and this will get many more areas covered. You also have satellite TV that can provide Internet through the satellite.
An area that looks promising is for your electrical companies to provide service through the electrical wires.
Fiber will give you the fastest, then wired cable and wireless will provide next fastest depending on how set up and DSL will usually follow in speed but has faster possibilities and satellite general is next and dialup is the slowest.
Although many areas have only one ISP, if you look around you may find more possibilities and many areas now have more than one ISP.