The Doctor and his travelling companion, retired army officer Alistair Lethbridge-Stewart, take a random trip in the TARDIS - and land on the planet Skaro. The Doctor helped the Thals to defeat the Daleks years ago, so he is surprised to find the Thal city under Dalek occupation. He determines to help them again, but what is the Daleks' purpose in keeping the Thals alive? Does it have anything to do with the Daleks' mythical creator, named in their teachings as Davros?
The Dalek race has split in half - one group abandoned the search for Davros and went off to conquer the universe while the second group continue to search for Davros. The first group are known as Renegade Daleks. The Doctor is more familiar with those who abandoned the search for Davros.
The Daleks plan to transform the Thals into Daleks to fight against the Quatch.
The Dalek/Thal war being caused by a third alien race, and Daleks being capable of coexisting with the Thals, were elements dropped from Terry Nation's original proposal for TV: The Daleks. 
The Thousand Year War and Davros' history are different (to the proper timeline), most notably that the war was considerably shorter than a thousand years and started closer to the end. The Quatch's involvement - and the Quatch themselves - do not seem to be caused by any difference in the Doctor's timeline.
Because of the ubiquity of this race, no attempt will be made here to list every appearance in every medium. Instead, we focus on audio appearances here, except for Dalek Empire, which has it's own template, and invite you to explore a complete list of appearances elsewhere.