ISPs do exactly what the the name describes: provide internet service to customers, in both residential and commercial environments. Internet service providers first emerged in the late 1980s and the early 1990s, when internet access first started becoming widespread.
Initially, telephone companies operated as ISPs, due to the prevalence of dial-up internet. Later, these companies expanded into broadband providers of DSL (Digital Subscriber Line). Then, in the late 1990s and early 2000s, cable broadband was introduced, and many cable television companies began offering internet service alongside TV and voice services.
Today, there are hundreds of different internet service providers, and they can be commercial companies, non-profits, or even publicly-owned utilities. Depending on where you are in the country, you may have a number of different ISPs to choose from to get internet service.