MySQL is an open-source relational database management system (RDBMS) that uses Structured Query Language (SQL) for managing and querying data. As one of the world's most popular database systems, MySQL powers everything from small web applications to large-scale enterprise systems, offering ACID-compliant transactions, replication, and high availability features. A critical distinction to understand: while MySQL supports multiple storage engines, InnoDB (the default since MySQL 5.5) provides row-level locking and foreign key constraints, whereas the legacy MyISAM uses table-level locking—choosing the right engine and understanding isolation levels can make the difference between a responsive system and one plagued by deadlocks.