I generally use the term "rude" to describe someone who doesn't care about other people's feelings. They just do whatever they want, even if it annoys or upsets people, without any real purpose to do so. Generally when the person is driven by feelings of superiority and/or malice. So I guess you could say, my interpretation of "rude" mostly applies to trolls.
In terms of following social norms: how you talk, how you dress, how you behave. Oftentimes, this is just a difference in culture, not actual "rudeness". Over the years, people have somehow twisted the definition of rude into meaning, "Oh, you're etiquette is different from mine", and they use it to portray you as a selfish person.
The irony is, they're the rude and selfish ones, because they're trying to conform you to their standards of living, the way they've been raised. And they show zero tolerance for anything that's different from what they know.