Apache Cassandra is a free and open-source, distributed, wide column store, NoSQL database management system designed to handle large amounts of data across many commodity servers, providing high availability with no single point of failure. Cassandra is optimized for write-heavy workloads and offers linear scalability and fault tolerance. It is used by organizations for a variety of use cases including real-time analytics, content management systems, messaging platforms, recommendation engines, and more. Cassandra's decentralized architecture and masterless design make it highly resilient to hardware failures and network partitions. It supports flexible data modeling and offers tunable consistency levels to meet the requirements of different applications. Cassandra is used by companies such as Netflix, Apple, eBay, and Spotify to power their mission-critical applications and services.