Warriors is a series about cats who live in the forest. There are six current main series. (Dawn of the clans, Warriors, A new prophecy, Power of three, Omen of the stars, Vision of shadows)
There are also a bunch of manga books and super editions.
They are writen by Erin Hunter.
The cats live in four clans, ThunderClan, RiverClan, WindClan, ShadowClan.
The main series take place in ThunderClan.
ThunderClan: fierce and brave.
RiverClan: clever and strong.
WindClan: swift and loyal.
ShadowClan: wily and proud.
The cats live under Silverpelt, (the Milky Way) and believe that all the cats that die go to StarClan and watch over the clans.
Instead of saying "Oh thank god!" they say "Oh thank Starclan!"
Here is a map over the territory:

And the new territory:

I will go deeper into different aspects and the clans in future chapters. This is just a basic overview. Tell me if I missed anything!