A building society is like a bank, but it's owned by the people who use it, instead of by shareholders. That means all the money it makes (like interest) goes back into the society and makes it better, not into the pockets of shareholders. Building societies are also more likely to lend money to people who need it, like to buy their first home, because the people running it are not just thinking about making money. They want to help their members, the people who use their services.