The Internet is the largest network in the world. BY FAR. The inner working are extremely complex. However, if you break it down . . . it is composed entirely of boxes and links. The boxes are routers or switches. The links are fiber, copper, or air (wireless).
The Internet is like a huge spider's web that encompasses the globe. All the slender fibers of this web are used as tiny highways for the flow of data. At each point where two or more fibers meet, there is a box to direct traffic.
People often confuse the WWW (WorldWide Web) with the Internet. But the WWW is merely an overlay on top of the Internet. The Internet offers connectivity - and that is all it offers. It is the higher level protocols that have created capabilities such as the WWW (WorldWide Web), FTP (File Transfer Protocol), TELnet, etc.
Where the Internet Begins and Ends
The boundaries of the Internet are not well defined. This is why diagrams usually show it as a cloud. It is like a giant ameba, that grows and consumes everything in it's path.
The Internet is composed of 3 main areas:
core - used for transit traffic, which means, all traffic is merely passing through. This traffic flows between core devices.
edge - used for access traffic and transit traffic. Access Traffic is the data traffic that flows to or from the end users. The transit traffic flows between the edge and core devices.
access (end users & end equipment) - the people that actually use the Internet, along with devices connected to the net, such as web servers, storage, etc.
Some people believe the the Internet begins and ends at the edge routers/switches. However, since the end users are addressable entities (they do have IP addresses), they are an inclusive part of the Internet.
For example, if you have a dial-up connection that you use for Internet connectivity - when you are not connected, you are not a part of the Internet. But when you dial-up and logon . . . you can now send and receive emails, create chat sessions, or even act as a web server if your ISP allowed it. You have become a part of the Internet. Similarly, when a LAN administrator provisions a router to give all users Internet access - that LAN and all of its users becomes a part of the Internet.
So, the end of the Internet is continually flexing and retracting as end users connect and disconnect, and as network administrators add routers, web servers, network printers, etc. Overall, of course - the Internet is constantly growing at an exponential rate.
Internet Tiers and Peering Points
The Internet is not a flat network . . . rather, it is hierarchical - or tiered. A Tier 1 provider, such as Sprint, AT&T, and MCI - has a large backbone, and peering points where it can transfer traffic with other providers. A Tier 2 provider is usually a large ISP, such as Earthlink - and typically offers direct connections to end users, has it's own backbone but not for major, worldwide traffic.
A tier2 provider relies on the Tier 1 providers for wide-area transport of IP packets, and for peering. The peering not only allows packets to be sent between two different providers - it also allows routes to be advertised to the world. To advertise itself, each device, with its own IP address, says "Hello world - here I am and here is how you can reach me !! ". It does this by sending out routing information to all providers via the peering points that interconnect them logically and physically.
NAP - Network Access Point
Also called an IXP (Internet eXchange Point), a NAP is a central hub for peering and traffic exchange between majpr ISP's.