Cassandra is a highly scalable, open-source distributed database designed to handle large amounts of structured data across many servers. It provides high availability with no single point of failure and was created by Facebook to power search on their messaging platform. Cassandra uses a decentralized peer-to-peer architecture and replicates data across multiple data centers for fault tolerance. It emphasizes performance and scalability over more complex query options and does not support features like joins typically found in relational databases. Companies like Netflix and Hulu use Cassandra for its availability, scalability, and ability to span large clusters with minimal maintenance.