Android Studio is built off of the codebase[0] from IntelliJ 13. The JetBrains products like RubyMine, WebStorm, and seem to be built off of the same core IDE with plugins for languages/platforms and features. So most of the functionality is really just in plugins, and of course IntelliJ probably offers additional Android development functionality for a price.

[0] http://www.jetbrains.org/pages/viewpage.action?pageId=983211