MariaDB
In plain terms
An open-source, community-driven fork of MySQL.
Going deeper
MariaDB began as a drop-in MySQL replacement after Oracle's acquisition, staying open and adding features. It's compatible yet independently governed.
Under the hood
Largely MySQL-compatible with extra engines and optimizations; favored by those wary of Oracle stewardship.
Examples
MySQL replacement, Linux distros' default DB, web apps, hosting.
Think of it like...
Like a community-run edition of a popular tool, keeping it free and evolving.