Centralized vs Decentralized Internet
Decentralized Internet can be identified using peer to peer connections rather than using a single point network access. The major difference between centralized and decentralized Internet is that the latter connects to different points (peers: other computers, printers, scanners, servers etc) without having to go through a mandatory point. There is no fixed network topology here. In fact, none of the network topologies, I learned so far, belong to decentralized Internet.
Coming to Centralized Internet, there is a mandatory centralized point (hardware). It is mostly a server as in client-server models. It can also be a hub or something through which, all the data on the network must pass. This model is an example of the current Internet state. You can find smaller networks inside bigger networks in a way that anyone wanting to snoop on the data can do so easily. For example, all data coming into the US or going out has to go through certain servers. This facilitates audit and scanning of the data whenever NSA or likes want.