Hello, to answer your question: 0.18 is a major release, and 0.18.1 is a minor release. Major releases add new content or make 'major' changes, while minor releases primarily focus on things like bug fixes.
You can tell this sort of thing based on where the numbers in version change. X.1.1, for example, has X being the release or complete version, with, often times, 1.0 being the game being completed (or near) and coming out as a release. 0.Y.1, then, would see Y as a major release. And 0.1.Z, therefore, would see Z as a minor release. Hope this explains the system, it is my understanding of it, though, so apologies if I got anything wrong.