It's along the lines of a Bethesda RPG - a huge open-world RPG (not quite, there is a 'world map' that ports you between 2-3 zones, but each are in themselves sprawling), in which you're free to follow the main quest as much as you like, but also to just head off exploring the countryside and do your own thing. As well as the main questline and significant secondary quests, the game features "Witcher Contracts" that you can find on village noticeboards and usually involve tracking down and killing a powerful monster, and also Treasure Hunts (which can either directly be for loot, or to find rare diagrams for crafting enhanced gear that can't otherwise be obtained).
As well as the crafting of weapons and armour, the game involves collecting alchemy ingredients to make potions and oils to use when facing specific types of creature. Geralt can increase his knowledge of which items to use when (found in the in-game bestiary) several ways, including finding books on the subjects.
It doesn't have distinct classes but there are skill trees in both weapons and 'signs' (rudimentary magic) that you can put points into as you level up to give you some specialisation fitting your playstyle. You also can't pick your character or appearance - you play as Geralt and have to put up with his past activities and relationships and reputation. There are a couple of potential romantic threads to unpick, including more than one significant one. There are dialogue options that can help or hinder quests, but there's no 'morality' tracker based on your responses or actions like you find in some RPGs.
And throughout it all there's Gwent, a deck-building game-within-a-game that sees you tracking down cards so that you can compete in tournaments (which are themselves part of quests). Most traders will play for money, but some important characters will play you for powerful cards they possess.
