An Internet service provider (ISP) is a company that connects individuals, businesses, and various types of organizations to the Internet. An ISP must possess the equipment needed to act as a point-of-presence on the Internet for the geographic area that it serves. Some ISPs are large national or multinational corporations which own or lease high-speed communications lines and offer access to many locations, while others are limited to providing service to a single city or region. Many small ISPs provide service through modems and high-speed digital communications networks (such as the Integrated Services Digital Network), while larger ISPs also may offer additional private-line hookups (such as high-speed T1 communications lines). Small ISPs generally hook into regional ISPs. A few well-known national ISPs are America Online, AT&T WorldNet, CompuServe, and Microsoft Network.

An ISP, also sometimes called an Internet access provider (IAP), sells computer access to the Internet, but will usually also provide such services as design, creation, and administration of web sites; access to newsgroups; training; and technical help. A customer generally establishes an account with an ISP, which then provides the user with a software communications package (bundle of applications to run on their personal computer), identifying username, password, and local or toll-free telephone number for access to the Internet. Once the software is installed on the user's computer and the registration process is completed, the user will be able to browse the Web, exchange e-mail, and establish a web site of their own if they so wish.

ISPs usually offer a packages for their customers. There are accounts tailored for large companies, providing a direct connection from the company's network to the Internet; there are also accounts for individuals who wish to use the Internet for personal or business purposes. These individual accounts are sometimes offered at a monthly fee for unlimited service. However, some ISPs charge for access time beyond a certain limit.

ISPs are connected to one another through a handful of network access points (NAPs). A NAP is a public network exchange facility where ISPs connect to one another in accord with so-called peering agreements. Such traffic-exchange arrangements are made by the larger ISPs as they interconnect with each other through ISP switching centers. The NAPs are a critical component of the Internet because the connections between them determine how traffic is routed.