Cassandra is an open source, distributed, decentralized, and fault-tolerant NoSQL database that is highly scalable and provides tunable consistency. It was created at Facebook based on Amazon's Dynamo and Google's Bigtable. Cassandra's key features include elastic scalability through horizontal partitioning, high availability with no single point of failure, tunable consistency levels, and a column-oriented data model with a CQL interface. Major companies like eBay, Netflix, and Apple use Cassandra for applications requiring large volumes of writes, geographical distribution, and evolving data models.