is OS X Mountain Lion update free of charge and is it better than OS X Mountain Lion 10.8.2?

Hi a family member just bought me an iMac 21.5 in, and as soon as I turned it on it asks if I want to udate it to OS X Mountain Lion 10.8.3. Now I never used a Mac before, and I'd like to know if I need this update because I have 10.8.2. Any difference in the numbers and is it free of charge?