Public in Classes

Hi, Just started with object oriented programming and glad I have come across it, just need a few things clearing up. When I make a variable or method public, does this mean that it can be used outside of classes, for instance in other classes or as well as instantiating classes? So if I made it private, it wouldn't be able to be instantiated or extended in other classes, am I right? I have never added public when I am creating methods so I presume its already set as default if you don't add it? Hope you can understand my question. Cheers.