The beta is more focused on polish and content. The aim is to add proper modding support via a stable API, some kind of non-intrusive narrative to the game to help drive the game experience early on, and a late game goal. Updates will be less frequent so as to make sure stability is maintained thanks to more extended testing. Despite this, there have already been two beta releases: client and server Beta 1.0 followed quickly by client 1.0_01.
The beta milestone also marks a change in the license for the game: the line that promises all future versions of the game for free has been removed. This change only affects people who buy the game as of today, so if you got the game when it was in alpha, you will still get all future updates for free. Buying the game during the beta will include all updates up until release, and bug patches will be free.