With the new patch you will have the option of resetting the skill level to 15 once it reaches 100. All perk points in the tree that you have spent will be removed from their assigned perks and will go into a pool of unused perks.
Your skill will reset to 15. You must increase your skill to purchase a perk with a higher requirement. You are in all ways a level 15 in that skill, just with some extra perks to spend.
All calculations are based on 15 skill, not 115. Your potions will be weaker, your attacks weaker, your enchantments weaker, etc. As you increase the skill you will gain new levels, can increase magicka/health/stamina, and gain additional perk points in addition to the ones you already earned and reset.
You can increase the skill to 100 and reset it again, gaining more levels (and therefore magicka/health/stamina and perk points) each time.
Seems like there are a lot of topics on this (so what's one more). Hope this clears things up.