Each board is like a different group of people, and none of the groups can see the other groups unless they are invited. All that is shared in the group, can only be seen by its members.
You can invite your friends by using a code that will be sent via email or Twitter, so all your people can quickly enter your boards.
The good thing about Glassboard is that it allows you to control what you share and with whom you share it. You can create groups with different purposes and with which to share certain files, for example your family or work group.
Unlike most social networks, in Glassboard you don't have to put up with any type of advertising.