Haven't seen any exact thresholds regarding the skill ranks besides the fact that master means that you have reached 100* in a skill. So perhaps it will be something like this:
0-20 Novice
21-45 Apprentice
46-70 Journeyman
71-99 Expert
100 Master
Those are just numbers I came up with right away.
*
SourceQUOTE(MrSmileyFaceDude)
Actually, you do gain special abilities as your skills advance. There are 5 skill levels that you can attain as you progress through each skill from 0-100: Novice, Apprentice, Journeyman, Expert, and Master (Master of a skill has 100 points in the skill). At each skill level, there are perks you earn that are appropriate to the skill. For the melee combat skills, for example, they mostly involve new power attacks that give you additional damage, and later, the chance to disarm or even temporarily paralyze your opponent. There are perks for all 5 levels of each skill.