Cassandra is a distributed database designed to handle large amounts of structured data across commodity servers. It provides linear scalability, fault tolerance, and high availability. Cassandra's architecture is masterless with all nodes equal, allowing it to scale out easily. Data is replicated across multiple nodes according to the replication strategy and factor for redundancy. Cassandra supports flexible and dynamic data modeling and tunable consistency levels. It is commonly used for applications requiring high throughput and availability, such as social media, IoT, and retail.