Apache Cassandra is a distributed key-value store intended to run in a data center and also across multiple data centers. Initially it was designed as Facebook as an infrastructure for their messaging platform. Later it is open sourced, and today it’s one of the most active Apache projects.
If you are using eBay, Twitter, Spotify or Netflix you are consuming data provided by Cassandra. For example, Netflix uses Cassandra to keep track of your current place in a streaming video, as well as movie ratings, bookmarks and user history of 90+ million users.