The boundaries between centralized and distributed architectures are getting fuzzier these days because a lot of companies are keeping their data on small servers while simultaneously moving all those servers to a central station. E ach architecture has different strengths and weaknesses. Distributed architectures arose partly because centralized mainframe models were cursed as inflexible and expensive. Zealots will argue about which model is better, but one thing everyone agrees on is that changing from one to the other is an enormous pain in the neck. The underwear of your computing architecture.
Nobody wants to see it, they just want it to function properly in the background.
It comprises the network wires and routing equipment, the operating systems, sometimes even the desktop PCs themselves. The server stores the data, the middle tier handles most of the processing and the clients display the results. Thin ones are more like dumb terminals that just repeat whatever the server tells them.