@Myrtle – The beta has to be a public beta, meaning it has to be connected to the community somehow but @Cecilia_Rosewood is correct, it can be open or closed… or (my preferred) you can do both, or even multiples of open and closed betas.
Everyone has a different way of getting to the development of a game… the goal here is to get some community feedback, so that you, as a developer get an actual idea of what the audience you will be selling your game to wants.