RxJS is a library for reactive programming that allows composing asynchronous and event-based programs using observable sequences. It provides the Observable type for pushing multiple values to observers over time asynchronously. Operators allow transforming and combining observables. Key types include Observable, Observer, Subject, BehaviorSubject, and ReplaySubject. Subjects can multicast values to multiple observers. Overall, RxJS is useful for handling asynchronous events as collections in a declarative way.