The Skill System in Rend is special, because you can Level nearly every Profession you can do in game.
These are categorised as Adventuring Skills, Crafting Skills and Combat Skills. Different to the Class System you can Level Up every single profession in the Game not only 2 out 4, but you will need a lot of time to level them all to max level. How to Level Skills:
The more often you do some of the professions the better you get at it. Refering to the Blog post: Punching trees gets you better at punching trees?

What happens when I level up a profession:
Simple The better you get, the better you get. So if you Lumber a lot you lumber faster and more items. For Combat its do more damage, faster reloads and less recoil.
BUT Rend has an unlock system not only a passive system!
At certain Levels you can Choose between some Perks these Levels are: 5, 10, 25, 50.
Also you Unlock better crafting recipes with a higher level. An Example for the Adventuring Skills:
At Level 5 in Herbalis you can Choose between "Friend of the Herbs" aswell as "Huntin for the Rare Bloom".
FotH gives you an increased yield by gathering plants
HftRB gives you a higher Chance of Rarer Resources.

Combat Skill example:
At Level 50 in Bow Fighting you can choose between "Hunter Perk" and "Sniper Perk".
Hunter Perk gives you a % boost on Movement for 5 seconds up to a stack of 5times.
Sniper Perk gives you a % dmg boost while standing still, up to 2%.

Hint: You can NOT reset Perks! Actual Adventuring Skills that have a Level: Lumber Stone Plants Creatures Metal Spirits Heat Cold Riding Spirit Realm Actual Crafting Skills that have a Level: Weapons Light Armor Heavy Armor Tools Magic Food Potions Research Repair Actual Combat Skills that have a Level: Wildlife Demons Players Bows Repeaters Crossbows Spike Launchers Light Dark Spirit Leather Silk Thanks to Iamacyborg for the Screenshots! Source: