There are two things that the witchers do unique to themselves. While mages cast spells, witchers utilize a unique form of minor magic known as signs. In the books these are described as being drawn in the air with the index and middle fingers. 

Known signs: 

The Sign of Aard– a telekinetic wave that can throw back, knock down or stun an opponent. This sign can also be used to destroy obstacles, for example crumbling walls or stacks of barrels.
The Sign of Igni– a gush of flames that wound opponents. When upgraded, the sign has a chance of incinerating opponents and has a larger area of effect. It can also be used to detonate flammable gases emitted by some bombs.
The Sign of Yrden– a magical trap placed on the ground, which will wound and immobilize opponents. The sign's basic level allows you to place a single yrden sign on the ground. After upgrading it, you'll be able to place up to three yrden signs at a time. These will work in unison, creating an impassible barrier.
The Sign Quen– a protective shield that lasts for 30 seconds at the sign's basic level. It absorbs all damage, and the upgraded version reflects 50% of the damage back toward to enemy. 
The Sign of Axii– a charm placed on an opponent. If the charm attempt is successful, the enemy will become your ally for a short while, fighting at your side.
The Sign of the Heliotrop–The Heliotrop Sign creates a temporary immobile circular force-field area around the witcher, which considerably slows down everything inside the field except the caster himself. 

The other unique ability is in their use of potions and alchemical mixtures. Because of the trials of herbs that they go through as children, they have a tolerance to chemicals and distilations which would prove fatal to anyone else. 

Several recipes have been designed (and discovered) specifically for witchers. Essentially these boost their abilities beyond the capacity of most living beings, such as granting extreme speed and nightvision while having the side effect of making them look like corpses. Things like that. Some seem like spell effects but most are just insane stat boosts. 

Oh! And I almost forgot their Medallions! A Witchers Medallion vibrates and let's them know whenever there are mgics or supernatural disturbances in close proximity.