Stalin, was a very paranoid person. He launched the purges just after the communist party had launched rapid industrialisation, and collectivisation. The word purge means cleanse. Stalin launched the great purges in the 1930's. He started with the purging of the communist party, the excuse he used was that he was getting rid of opportunists who may have been members of the party. Many of the people who were purged from the party were simply expelled from the party, but for some more extreme action was taken, some were sent to the Gulags (a prison camp) or even executed (shot).
During the 1930's Stalin became increasingly interested in the growing population of his only real rival Kirov. At the 1934 party congress Kirov only received 3 negative votes, the fewest of all of the candidates, whereas Stalin received over 1000 negative votes. In 1936 Kirov was mysteriously murdered, this was very convenient for Stalin. During the show trials Trotsky , Kamenev and Zinoviev stated that they had played part in his murder (although we know they were being black mailed into saying what ever Stalin wanted them to say , as they feared what would happen to their family). After Kirov's , murder Stalin passed a new law which stated that terrorist organisations or terrorist acts were to be investigated in no more than 10 days and executed quickly.
Many military officers were also purged, this included a large number of officers from the red army. In August 1940 Trotsky was executed. This marked the elimination of the last of Stalin's previous rivals. This left only 3 old Bolsheviks remaining and this included Stalin.
Stalin didn't just purge people of power though, he also executed many ethnic minorities , which included Germans and Americans who had fled to USSR during the Wall Street Crash and The Great Depression that was taking place in the West. Most of who were captured , were executed in the Gulag prison camps, only a lucky few survived.
Stalin forced propaganda on the people of the USSR. He tried to re-write history in text books so that children learnt things he wanted them to learn. Many people Stalin had purged were wiped from history as photo's of them were destroyed or they were cut out of photos, their names were took from text books, as if the never lived. And history of the October revolution 1917 was re-wrote as a story about Lenin and Stalin.
In the end it was known that Stalin purged , teachers , home makers, priests and musicians.
Stalin was said to have signed, 357 lists of people who were waiting to be executed which had over 40,000 names on, which were to die in one day. Stalin was reported to have said no - one will remember the riff raff in ten years time any way.