In the old days we called it client/server technology. Client/ server existed even before networks. How is that... without a network? That was the days of sneaker net. We used removable disk drives to transfer the data. Hence the name... Tennis Shoe Net never caught on.
Client/Server really took off when networks came on the scene. Data could be transferred in real time. Token ring was a kinda flop but Ethernet became the main stay real fast. Networks begat networks and the whole ball started rolling.
Today we have the web... Although, if you break everything down it's really the same thing as the old client/server technology with one really big difference... Standardization. While the old client/server paradigm came in as many flavors as there are programmers building them... the web uses HyperText Transfer Protocol (HTTP) which... in a basic way... standardizes everything. Now a days the flavors are in the frame works. But that's a different story.
Web Server Architecture...
There are three basic types of web server models.
Processed based servers - multiple single threaded processes each of with handles one request at a time
Thread based servers -
Hybrid (Process and Thread) based servers
or prefork based servers have multiple single threaded process running in which each thread handles a single request. The biggest advantage of process based servers is stablity since the processes are independant. A problem process can be restarted without affecting other running processes. Its disadvantage is performance.
based servers employ a single multithreaded engine. Each thread handling one request at a time. This spawns a big advantage in that it uses less memory because threads share the same memory space. In addition they can share resource cashing. With that in mind stability becomes a big disadvantage because one problem thread can crash the server (since they have common memory addressing).
servers have multiple multi threaded processes where each thread of any process handles one request at a time.