Mac OS X 11?

Ok, I have a Mac OS X ( also referred to as a cheetah, I guess? im still kind of confused on all the cats) On my updates, there is an update that is called mac os x11 or something, but I thought that leopard was the latest version and was 10.5...? im kind of confused, is there a way I can upgrade my osx to something better? im sorry if this is confusing and stuff..