Observable types:

Flowable<T>:

Emits 0 or n items and terminates with an success or an error event. Supports backpressure, which allows to control how fast a source emits items.

Observabl<T>:

Emits 0 or n items and terminates with an success or an error event.

Single<T>:

Emits either a single item or an error event. The reactive version of a method call.

Maybe<T>:

Succeeds with an item, or no item, or errors. The reactive version of an Optional.

Completable:

Either completes with an success or with an error event. It never emits items. The reactive version of a Runnable.