The Master is, like the Doctor, a Time Lord who comes from the Planet of Gallifrey. The First Master which faced The 3rd Doctor was a Charming but very deadly man. He used Aliens to help him almost Defeat the Doctor like The Nestene Consciousness, The Daleks, Ogrons, Azal, Axons, Sea Devils, Kronos and even an Alien Mind Parasite. He used the Nestene but killed the Autons and then when the Sea Devils captured him he then blew up their lair.
The Second Master had used up all 13 of his Lives so he was an Ugly Monster which was a Skeleton Form. This Master Faced the 4th Doctor on Gallifrey and Worked with a Senator Time Lord. In the end The Master killed him and many Other Time Lords. He then Nearly Destroyed the Whole Gallifrey but the Doctor has Stopped Him Once Again. The Third Master also Face the 4th Doctor and took Control of a Very Ancient Power Force on the Planet of Traken. He was then Defeated again but he took over a Body of a Trakenite Man.
The 4th Master Faced the 5th, 6th and 7th Doctors in his Battles. He had tried to kill them and had worked with some Villains like The Rani and Cybermen. He was always defeated and when he had the End of his New Body he was Leader of the Cheetah People and was Becoming One as he was changing into one of their Natives.
Then The Master returned Once More with a New Body as he Faced the 8th Doctor. As Dalek Exterminators couldn't stop The Master. He Lived on an Alien Snake that soon took over Bruce. He tried to steal the Eighth Doctor's Remaining Life Force but he was Stopped Once More for a While.
He has appeared in 3rd Doctor Season 8: Terror of the Autons, The Mind of Evil, The Claws of Axos, Coloney in Space and The Daemons.
Season 9: The Sea Devils and the Time Monster.
Season 10: Frontier in Space.
4th Doctor Season 14: The Deadly Assassin.
Season 18: The Keeper of Traken and Logopolis.
5th Doctor Season 19: Castrovalva and Time-Flight.
Season 20: The Kings Demons20th Anniversary Special: The Five Doctors.
Season 21: Planet of Fire.
6th Doctor Season 22: The Mark of the Rani.
Season 23: The Trial of a Time Lord Episode: The Ultimate Foe
7th Doctor Season 26: Survival.
8th Doctor: Dr Who the TV Movie.