Discipline is pretty much the color. Honor, Discovery and agriculture for example. Each discipline has its own jobs though which are the professions. And they get experience/levels in discipline rather than the profession.

Sometimes traits say clans will get upset if they get a profession in a discipline, but apparently even if they only have the discipline and no profession they can get upset already.

There are 6 disciplines (Honor, Discovery etc.), each with many professions. When your clans gain experience in disciplines overtime, not profession; in other words, someone can get really good at crafting by being a logger, then you can decide to switch jobs and send him to be a stonecarver, for example. You DO lose all experience when you switch to a entirely different discipline. Think of it as the difference between career and specific job/position.