The easiest way to get to know the witcher is by playing the games. But essentially, the need to know details are the following.
-Witchers are monster hunters. They drink toxic potions to deal with(i.e. heal) themselves. 
-Monsters from all sorts of mythos hide in the wild and/or people's houses (though they are primarily focussed on european mythos). 
-Gwent is a card game in the witchers world.(here you might pick up things even more vague than my description.)
-The people all have pretty poor character.(Just in case you were wondering) 
-Kings and queens are basically at a constant state of war.
-Elves and dwarves do live among humans, though their numbers are significantly less and they therefor form a rebel/guerilla faction. 
-Mages are often feared/hated. There's a council of mages that seeks political power, but ultimately never seems to achieve it.
-There's various kingdoms, but really.. If you know the names Nilfgard(Southern kingdoms), Temaria(northern kingdoms), Skaal(oversees western kingdom) and "Scoia'tael"(<=elves and dwarves from everywhere), you've got the biggest players down. 
Hope it helped. 
Here's a link if you really want to read up on things(As I'm certain i oversimplified things for the sake of time-constraint.)

[url]http://witcher.wikia.com/wiki/Category:Kingdoms[/url]