Honestly? I don't think there is a difference, between the two. A "character class", being an adventurer, is nothing more than another profession, a job. One that can potentially yield much greater rewards than, say, being a blacksmith or an alchemist, but one that also carries a much greater risk of death. Potentially by gruesome means, sometimes by very embarrassing means.
In short, I believe the distinction between a 'character class' and 'character profession' is purely a gameplay distinction, and one is only not as developed as the other, because the game focuses more on the journey, the action, rather than the intricacies of mithril smithing, or vast possibilities of lotus leaves usage in potion-making.