The Internet is a series of connected networks designed to allow high-speed communication between various organizations, companies and countries. The basic structure of the Internet is broken up between tier 1, tier 2 and tier 3 carriers.
Tier 1 networks are backbone Internet providers that connect directly to the Internet. These companies have agreements with one another to allow for the free passage of traffic between them. Internationally, there are five Internet service providers (ISPs) that are considered tier 1. These are AT&T , British Telecom, Equant, InfoNet and MCI (now Verizon).
The next level of ISPs are tier 2. They are smaller, mostly regionally based, and they tend to provide ISP links between larger organizations. Tier 2 providers have peering agreements with the larger, tier 1 providers, but generally don't peer with each other. Examples of tier 2 providers include Level3, Sprint, and XO Communications.
The smallest level of ISPs are tier 3 providers. These providers generally supply Internet connectivity for homes and small businesses. They are regionally based and often peer with larger tier 2 and tier 1 providers to allow global Internet connectivity. Examples of tier 3 providers are Time Warner Cable, Verizon Fios, Google Fiber and Cablevision Optimum.