LWN.net has a nice article on their front page on Drizzle's and MariaDB's recent beta and RC releases. it is behind a paywall for a few more days, but using the link below you can already read it.
For years, MySQL has been the highest-profile open source relational database system, but with the Sun (and, later, Oracle) acquisition of MySQL's corporate parent MySQL AB, the development community has split in several directions. Now, a few years later, both of the leading community-driven forks of MySQL, Drizzle and MariaDB, have made important new releases.
While Drizzle is an attempt to hone the MySQL code base into a lean-and-mean database manager, MariaDB takes nearly the opposite approach, building a system with an array of high-end options well suited for enterprise usage.
I guess on a high level that's pretty accurate.