So, where does Firebase fit into all of this? Firebase is a NoSQL database that stores data as simple JSON documents. We can, therefore, compare it to other document-oriented databases such as CouchDB (which also stores data as JSON) or MongoDB (which stores data in the BSON, which stands for binary JSON, format).
Although Firebase is a database with a RESTful API, it's also a real-time database, which means that the data is synchronized between different clients and with the backend server almost instantaneously. This implies that if the underlying data is changed by one of the clients, it gets streamed in real time to every connected client; hence, all the other clients automatically get updates with the newest set of data (without anyone having to refresh these clients manually).
So, to summarize, Firebase is an API and a cloud service that gives us a real-time and scalable (NoSQL) backend. It has libraries for most server-side languages/frameworks such as Node.js, Java...
