The PC version of The Witcher 3: Wild Hunt has some rather deep (even if not the deepest I've seen) customization options for graphics detail, divided in two menus, graphics and post-processing.

The the graphics details have four levels of presets, Low, Medium, High and Ultra. The post-processing options have three levels: Low, Medium and High. Since High doesn't include HBAO+ ambient occlusion, for the comparison below I associated to each level of graphics the level of post-processing with with the same name, and added HBAO+ manually to Ultra graphics.

Of course you can fine tune your experience more, as those listed above are just presets. For instance, I immediately deactivated motion blur while playing (not for this comparison), which is an effect that I personally hate in games, and eats needless resources.

You'll notice a large difference in Geralt's hair between Medium and High. High activates Nvidia Hairworks for Geralt. Ultra activates it for all characters.

Of course, if you want to better enjoy the differences between each screenshot, you should click on them to see the full resolution version, then flip between different tabs of your browser to check out the gradual increase in detail.

Ultra

High

Medium

Low

Ultra

High

Medium

Low

Ultra

High

Medium

Low

Ultra

High

Medium

Low

Ultra

High

Medium

Low

While the differences are visible, the game is still definitely pleasing to the eye even at Low settings, ensuring that even those who don't have an extreme gaming PC can still enjoy Geralt's new adventure.

That said, if you have to play on Low, you might be better of playing on console, unless you enjoy keyboard and mouse more than a controller and the mods that will inevitably come for the PC version, of course.