In the first Dalek serial, commonly known as The Daleks (1963), it is explained that the planet Skaro was once home to two humanoid races, the peaceful and scientifically-advanced Dals and the warlike Thals. Following a short but terrible nuclear war between the races, the Dals were mutated and became the aggressive and xenophobic Daleks.
This information is contradicted in the 1974 serial Genesis of the Daleks, in which the Doctor returns to Skaro before the creation of the Daleks. In the later serial, the race are called Kaleds and have been involved in a generations-long war of attrition with the Thals, not the short nuclear winter described previously. (The serial also suggests that the Kaleds' mutations were not as bad as implied by the earlier account, and that their development into the Daleks was engineered by the Kaled scientist Davros for his own purposes.)
All later serials treated Genesis of the Daleks as the true story and used "Kaled".
Several theories exist to explain the discrepencies, outside of the real one in which different production teams simply re-wrote Skaro's history. One theory holds that the account in The Daleks, which comes from centuries-old Thal oral history, is simply wrong. Other more complicated theories, making use of the time travel aspect of the series, have also been suggested.