In Witcher  there are a few core mechanics to be aware of, for every game including Witcher III. First, a little background. Just to understand where the Witcher comes from, it comes from a series of books that have been around for awhile. There is a lot of source material and reasonings behind everything in the game. So that's the first thing to understand, even from the first one game, you're probably not going to really understand the world without appreciating the books but that's okay - the game is fun and does a pretty ok job with letting you know what is going on. 

Moving on - there is a core mechanic to every Witcher game. The swords. Geralt has a silver and steel sword. Steel is for man and silver is for beast. It gets a bit more complicated than that. Steel works against humans and humans only. Steel and rune swords work against monsters (Geralt is a monster hunter himself). 

Speaking of runes, rune swords can be equipped with runes which modify the weapon and add additional damage. Three runes forged on top of a silver sword make a rune sword. Steel swords can't be upgraded in this way, instead you'd craft a meteorite sword, which has additional human fighting properties. 

Steel doesn't work on monsters and silver doesn't work on humans. This is the basic foundation of Witcher combat. 

Signs are the magic casting in Witcher. They do the following, usually universaly throughout the series: 

• Aard: Knockback. 
• Igni: Flame attack. 
• Quen: Shield. (In Witcher II it is more powerful in the rain) 
• Yrden: Ground trap. 
• Axii: Charm. 
• Heliotrop: Ground shield / slow. (Not included in Witcher I w/o mods). 

Character statistics are usually universe. Vitality is health, it's restored with mediating or a variety of potions (Rubedo), Swallow potions, or food. Endurance is used to cast signs and to block with your fists. Stamina increases Endurance, along with Tawny Owl and any drink that isn't liquor. 

Toxicity is one that might trip you up. Everytime you quaff a potion, your toxicity increases. This can be decreased through meditation. As your toxicity increases, the damage increases as well. At around 2/3rds your bar, he takes health damage. At near max he can't run. At 100% he dies. 

Crafting is a big part. You only need two things to craft something. The first is a diagram and the second is the components. Then you take them to a craftsman in town who will help you out in crafting it.  

Inventory space is limited in Witcher. I suggest modding it bigger and just enjoying the game, but if you want to keep the default, you can store your items in chests available in different locations. Usually items in the chest move chapter to chapter. 

Questing is a bit different between Witcher I & II and III. In I & II you follow chapters, but in III it's more of an open world where you can explore (ala The Elder Scrolls) and complete side quests, in addition you can pause any quest at any time and go complete another quest. The world will wait on you.  

Speaking of quests, universal throughout the series, your actions will determine the results of the ending and can have major impacts on the story. In Witcher II for instance, you will decide which map you move to next and have a completely different story, whereas in Witcher III the world is a lot more alive and actions have a bit more deeper consequences. 

Witcher sense can give you clues about your current quest. Don't hesitate to use it often!

That's about all for basic Witcher mechanics! Go forth and hunt monsters.