Couchdb is a database system, where entries are json objects (allowing attachments), and that you can access and synchronize via simple http requests.

As explained in wikipedia:
> Apache CouchDB is an open-source document-oriented NoSQL database, implemented in Erlang.

And as described in the couchdb official website:
> Seamless multi-master sync, that scales from Big Data to Mobile, with an Intuitive HTTP/JSON API and designed for Reliability.