According to http://store.steampowered.com/earlyaccessfaq, it is a system that allows you to gain access to games that are being developed with the community’s involvement. It also says that you will get the final game when it is released, but it’s up to the developer to determine when they are ready to call the game complete.
By joining the early access program, you get the chance to test early versions of the game that may or may not be fully functional, due to the fact that it’s a constant work in progress. You also get access to new features as they are implemented, and you also get to be heavily involved in the process of finding and ironing out bugs. Finally, when all that is over, you’ll get your copy of the completed product, for no additional charge.
Castle Story already has many features implemented, but there is still a lot to do before it becomes a complete game.
Also, note that the linux version is still a really old build and we will create a new one for this platform later. Thank you!