I like this overview.

The gameplay loop is what unites all games and game engines, although it may be veiled within the engine, replaced by abstractions or handler functions (callbacks). (Part 1.1)

Incidentally, in my view, it reproduces the common belief that a game has to be structured as a main loop that might be hidden to the programmer in which case s/he has to associate actions(handler functions) to events. This example attempts to prove that it’s not always the case.