Project Reactor is built with two core types based on the number of elements they process. They are considered as main building blocks to create a Reactive System using Reactor. They are Flux and Mono. They both implement the Publisher interface and conform to Reactive Streams specification, and are furnished with reactive-pull and back-pressure facility. They also have several other useful methods. Let's explore the details as follows:

Flux: It can be considered the equivalent of RxJava's Observable and can emit zero or more items, ending successfully or with an error signal. In short, it represents asynchronous event streams having zero or more elements.

Mono: It can emit, at most, one element at a time. It is equivalent of the SingleandMaybe Observable type from the RxJava side. A Mono type can be used for one-to-one request-response model implementation; for example, a task wish to send a completion signal can use a Mono type reactor.