ISP, POP, NSP, NAP, MAE, IXP

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The following diagram is an overall architecture of internet.

You home PC connects to a home LAN, your home LAN then connects to a tier 3 ISP, the tier 3 ISP have access to a NSP's network infrastructure (fibers, routers etc.) via a POP. These NSPs have peer to peer connections via NAP or IXP, so that the internet made of inter-connected NSPs span the whole globe.

ISP and NAP

• Local Loop also called "last mile", is a copper or fiber cable that connects the demarc to the closest switching office, called a central office. In the diagram, the local loop is a T1 line.
• Internet Service Provider (ISP), An Internet Service Provider (ISP) is a company that provides Internet access. The most common ISP is the provider who delivers Internet to your home or business for a fee. 
• Central office (CO) also called point of presence (POP), connects the customers to the provider's switching network. It is the entry point to the WAN cloud and the exit point from the WAN for called devices. The tier 3 Internet service provider (ISP) often has multiple point-of-presence in major cities, which are connected together by the leased lines from NSP.
• Network Service Providers (NSP) A network service provider (NSP) is a company that owns, operates and sells access to internet backbone infrastructure and services. They often sell network to tier 3 ISPs, or act as ISP by themselves. NSPs build and maintain the fiber optic cable and core routers. Some of the large NSPs are UUNet, CerfNet, IBM, BBN Planet, SprintNet, PSINet, as well as others. These networks peer with each other to exchange packet traffic via NAP. 
• Network Access Points (NAP) is a public network exchange facility where Internet Service Providers (ISPs) can connect with one another in peering arrangements. The NAPs are a key component of the Internet backbone because the connections within them determine how traffic is routed. They are also the points of most Internet congestion.
• Metropolitan Area Exchanges (MAEs). MAEs serve the same purpose as the NAPs but are privately owned. NAPs were the original Internet interconnect points. Both NAPs and MAEs are referred to as Internet Exchange Points (IXs).
• Tier 1 ISP, Tier 2 ISP and Tier 3 ISP, ISPs are categorized by their size. Tier 3 are generally small ISPs who only purchase their connection to the Internet from NSP. Tier 1 are generally internet giant NSP who owns so much network infrastructure that they don’t need to purchase any connection from other ISPs. These major players “peer” with other to allow the traffic between their networks to flow back and forth. Tier 2 are providers who have some interconnection agreements, but who also purchase some connections. Largest Tier 1 Internet providers are the networks that provide the backbone of the Internet. These providers build infrastructure such as the Atlantic Internet sea cables. These infrastructures allow the data exchange between continent and countries. Examples are Hibernia Networks, Cogent Communications.