A global system of connected computers that are located in over 190 countries throughout the world.
The Internet itself is a decentralized medium since each node (computer) is independent. This differs at the services level which are mostly centralized.
There are 3 accepted stages of the Internet:
Web 1.0 - early users are content consumers with most of the medium being text. Web pages were static for the most part.
Web 2.0 - interaction became two way with web pages becoming dynamic. With the introduction of social media, content consumers also became creators. In addition to text, video, audio, and photos were added. Mobile is also considered a key piece to Web 2.0.
Web 3.0 - decentralized data storage at the services level as compared to the centralized server system. Users have ownership over their accounts. The architecture is being built upon blockchains. Cryptocurrency is the first widespread application of Web 3.0.