Beta testing is the final stage before the game is released. When a program is first written it is tested only by a few people, usually all within the company. That's called an alpha release. It is then tested by a larger group of people who look out for bugs and make suggestions for possible improvements. That's the beta release. Sometimes a company can make several beta releases available in fairly quick succession for re-testing as bugs are fixed and areas are polished up. The main advantage of the larger group is that they can find out how it runs on a variety of computers, and it is more likely that bugs will be spotted.
Some beta tests are "open betas", which anyone can join in so long as they don't mind using unfinished software. LDW, though, uses "closed betas", where they invite the testers individully. See the
Beta Testing thread for more information.
Don't get
too excited because it's in the final stage and expect VV4 to be out tomorrow! There really is no way for the developer to tell when a beta test will be over, because it depends a lot on bugs found and their severity. I've taken part in several beta tests for other software, and the testing periods ranged from a couple of weeks to over a year.