An Internet Service Provider (ISP) is a company that charges a fee to provide you with Internet access. Customers usually connect to an ISP using a phone line (dial-up) or broadband (cable or DSL) connection. ISPs generally offer multiple options for connecting to the Internet. These options are characterized by different speeds and prices. Large ISPs in the US include Comcast Xfinity, AT&T, and Time Warner Cable.
From the blog: What is an IP address?