MariaDB is a RDBMS with all features as of MySQL and also created by developers of MySQL !
Yes, it is a replacement to MySQL and applications are being developed by MariaSB since they are :
- Open source
- Less bugs
- better performance than MySQL
MySQL is also open source but enterprise editions are not open source and is governed by Oracle.
MariaDB is a forked version of MySQL ,i.e copy of package and it was developed over concerns when MySQL was acquired by Oracle.
What is fork :
MariaDB is OpenSource and compatible with MySQL
Find more features here in official doc :