MySQL is an open-source relational database management system (RDBMS), which stores data in linked tables and is queried with the SQL language.
Created in the mid-1990s by the Swedish company MySQL AB, co-founded by Michael Widenius (the name comes from his daughter's first name, My), it was acquired by Sun Microsystems in 2008, then came under Oracle's control when Oracle bought Sun. It is distributed under a dual license: open source (GPL) and commercial.
MySQL has historically been tied to the web: it is the M in the LAMP stack (Linux, Apache, MySQL, PHP) and WordPress's default database. It is easy to install, fast for reads and offered by every hosting provider and cloud.
After the Oracle acquisition, Michael Widenius launched a fork, MariaDB, which is largely compatible. In new projects, PostgreSQL is often preferred for its richer feature set.
Why it matters when hiring
Almost every back-end developer has used MySQL, which says nothing about their database skills. Real expertise shows in schema design, indexes, optimizing slow queries and transactions. In the interview, have the candidate analyze a slow query: a good candidate talks about the execution plan before talking about a bigger server.