The event system is like the Hook system in Garry's Mod. It allows you to register functions on classes to receive a callback on certain global events.
[Event] attribute will add it to the list to be called.
The various events called from the engine currently:
Called clientside every frame to process input and encode outputs into a user command.
Called when a client disconnects.
Called just before the beginning of a rendering frame.
Called every server or client tick (60 times a second by default).
Called after each physics step, this is usually every tick but can be more depending on PhysicsSubSteps.
Called each time your C# is hotloaded after successful recompile.
You can register your own custom event callbacks and call them.
Events can pass any amount of arguments too.
If you want, you can also make your own custom EventAttributes (like
[Event.Tick]) and use them to register handlers. The behavior is the same as raw event names but they have all the benefits of a type (picked up by IntelliSense, checked at compile time, etc.)