Witcher 3 cheats (Witcher 3 console commands) work only on PC through the debug console. Commands fill out your gear, money, monsters around Geralt, or move you across the map.
How to enable the console
Close the game first. In Steam for The Witcher 3 pick Manage and Browse local files. In the game folder go to bin/config/base and open general.ini in Notepad.
Under the line ConfigVersion=5 add on a new line:
Line for general.ini and dx12user.settings if needed
On the next-gen version in DX12 that is not enough for some players. Also open Documents/The Witcher 3/dx12user.settings, find the [General] section, and add the same line there. Without both edits the console sometimes never shows up.
After you launch the game try the tilde (~) key above Tab. If nothing opens try F2, at sign, or apostrophe depending on your keyboard layout. The console opens as an empty window, you type the command and confirm with Enter.
Most codes want the item name in apostrophes inside the additem command. Commands are case-sensitive. When the tilde still does not work after you edit both files, the Debug Console Enabler mod from Nexus Mods helps.
My take on cheats
I would save cheats until after you think about why you want them in the game. Usually you can mess up your playthrough and after you turn on the console you lose Steam achievements. The game remembers that in your save.
It mainly makes sense in two situations. The first is when you are stuck and cannot continue the legit way, for example because of a quest bug or a broken save. The second is when you already beat the game, know the content, and just want to try some artificial build, a fight with a summoned monster, or a quick mod test.
Otherwise I would play the normal way. The review explains why The Witcher 3 is worth your time even without the console.
Cheat categories
Pick the category you are looking for right now. On the card you find a table of console commands with copy. Crafting and NPC spawn have their own tables for schematics and characters.
Conclusion
Cheats in The Witcher 3 are a tool for PC players, not a replacement for playing the game. You enable the console by editing general.ini and optionally dx12user.settings, then you pick a category from the list above and you find the specific command in the table on the subpage.