Types

Observable - Returns multiple responses. It uses onNext() and onError()

Single - Only ever returns one response. This can either be an onSuccess() or onError(). Either way a response is only called once where in the subscription is disposed.

Maybe - Either returns one response or no responses. It behaves the same as a Single except it could never return.

Docs

I've been using this in the context of RxSwift, but it holds true across Rx implementations.