My non-expert opinion based on very recent research has led me to understand it this way.
It all started with MySQL, but was ultimately bought by a competitor product. There were concerns about development and the company "promised" to continue it. However, the original guys behind MySQL eventually quit and started their own; MariaDB.
This last one has improvements and in "general" surpasses the capabilities of the former. However, the "rub" comes in third party support. MySQL has been around longer and thus you will find "easier" (as in more frequently) examples, code, etc than those for MariaDB.
So, from my perspective, if you have control/choice, I would go for MariaDB if you are intending to develop most of the code. If you are going to copy-paste, continue using "ready-made solutions" (LAMP/WAMP, etc), then you will find the journey less frustrating sticking with MySQL.