So what exactly do they do? That is a tough one, unless you understand the intricacies of MySQL, relationship databases and forks. Not the ones you eat with, but the software development kind. Basically MariaDB is an independent copy of MySQL (fork) that was started by ex-MySQL execs over concerns that Oracle, who acquired MySQL, might take unexpected directions with the development.
MariaDB was started by Monty Program AB from Sweden but they later merged with SkySQL in April 2013, and by doing so - reunited the key members of the original MySQL AB team.