Every time Geralt levels up after accumulating the required experience points, he gains one talent which can be allocated through the character development screen (accessible by pressing the key or through the meditation menu). On hard difficulty or higher, talents can only be allocated during meditation.
There are a total of fifty-one skills, each with two levels, into which Geralt may invest talents, and some of the skills can be further enhanced or 'mutated' by applying mutagens to them. The skills are divided into four trees, only one of which is initially available. After six talents have been allocated to skills in the Training tree, the remaining three skill trees are unlocked. The Magic, Alchemy, and Swordsmanship trees contain fifteen talents, with two levels in each of them. Since the game has a level cap of thirty-five (and you start at level one) you will only have a total of thirty-four talent points to distribute.
Every main tree contains one advanced skill which introduces a new gameplay mechanic called 'Adrenaline' that governs its use. The skills and their Adrenaline fueled counterparts in each of the talent trees are described on a separate page.
For skills that indicate a reduction of damage %, this is actually a percentage modifier of armor. The terminology can be confusing. So with 100 armor, 20% damage reduction, and a 300 damage blow, total damage received would be 300-(100*1.20)=300-120=180. Damage reduction modifiers do stack (10%, then 15% total to 25%).
The only exception to this is the parry description, where the % affects actual damage. So a 300 damage blow would only land 150 points through a level 2 parry, less any armor reduction. Also note that parrying means blocking in the Witcher.