Hello Doo,
An event is an instanceable unit of sound content that can be triggered, controlled and stopped from game code. Everything that produces a sound in a game should have a corresponding event.
A bank is a collection of events from your FMOD Studio project, formatted and compressed for use in your game. Banks allow you to choose which content is loaded into memory at any given time.
A bank can contain as many events as you need. Keep in mind that a bank must be loaded into memory, so banks that contain lots of events will take up more memory.
Thanks,
Richard