HBase is a NoSQL database that stores data in HDFS in a distributed, scalable, reliable way for big data. It is column-oriented and optimized for random read/write access to big data in real-time. HBase is not a relational database and relies on HDFS. Common use cases include flexible schemas, high read/write rates, and real-time analytics. Apache Phoenix provides a SQL interface for HBase, allowing SQL queries, joins, and familiar constructs to manage data in HBase tables.