To provide a community developed, stable, and always Free branch of MySQL that is, on the user level, compatible with the main version. We strive for total interoperability with both our own, and our upstream, communities.
MariaDB is kept up to date with the latest MySQL release from the same branch. For example MariaDB 5.1.47 is up to date with MySQL 5.1.47, and so on.
We do a merge from the main MySQL branch for every new MySQL release or when there is some critical bugfix applied to the main branch.
We strive to keep our main trees as free from bugs as possible. It should be reasonably safe to pull from our trees at any time.