The Holocaust in general evolved from being a sort of informal thing that was not particularly organized to being a well organized killing machine in which the victims were killed efficiently in large groups.
At the beginning of the Holocaust, the killings were carried out by special groups called Einsatzgruppen. These killings were done mainly in the parts of the Soviet Union that the German army conquered. These murders were generally done by shooting the Jews and burying them in trenches.
These murders, however, were too hard on some of the people who committed them and were in any case not efficient enough to kill the millions of Jews that the Nazis wanted to kill. Therefore, the Nazis developed the death camps in which people were killed through the use of gas and then cremated. This was much easier and could kill many more people much more efficiently. This was the most "evolved" stage of the Holocaust.
Don't forget the stages prior to the Einsatzgrupen. First stage discrimination. Second stage deportation. Einsatzgrupen was the third stage. Final solution was the forth.