These are both server database management systems used for Web development.  They will store data for you in tables and then provide it when asked.  MySQL has traditionally been used by Web developers but there are some recent issues relating to the openness of the code.  In response  MariaDB was created to replace it.  Everything you try in these tutorials will work in both.  MariaDB was designed to be a simple replacement rather than to introduce new ideas.  XAMPP currently uses MariaDB but you might find that some Web servers still use MySQL. 

Don't worry about all this but if you get the choice go for MariaDB.  Meanwhile if you see reference to MySQL or to MariaDB assume they mean the same thing.

The other main SQL Web database is Oracle.