Cassandra is a decentralized, highly scalable NoSQL database. It provides fast writes using a log-structured merge tree architecture where data is first written to a commit log for durability and then stored in immutable SSTable files. Data is partitioned across nodes using a partitioner like RandomPartitioner, and replicated for availability and durability. Cassandra offers tunable consistency levels for reads and writes. It also supports a flexible data model where the schema is designed based on query needs rather than entity relationships.