Apache Cassandra is a high performance, extremely scalable, fault tolerant (i.e. no single point of failure), distributed post-relational database solution. Cassandra combines all the benefits of Google Bigtable and Amazon Dynamo to handle the types of database management needs that traditional RDBMS vendors cannot support.
Take a quick look at the basics in this overview provided by DataStax, the developer of DataStax Enterprise, a distributed, scalable, and highly available database platform that delivers optimal performance either on premise or in the cloud for modern enterprise applications that manage both real-time and analytic workloads.
To learn more about Apache Cassandra and access free courses visit: https://academy.datastax.com/courses/ds201-foundations-apache-cassandra