The Yeti are big furry beasts who live in the mountains of the Himalayas. They are known as your abominable snowmen. The Yeti are actually robots created by the Great Intelligence who wanted to invade Earth.
When the Doctor first encountered the Yeti he was in Tibet in 1935. A man called Professor Travers wanted to capture a Yeti to add to his collection. The Doctor was captured by monks from the monestry. The Doctor's two companions Jamie and Victoria were being chased by a Yeti up in the mountains. Jamie however made the roof of the cave they were in fall onto the Yeti. Jamie thought he killed it as him and Victoria discovered these silver spheres. The Yeti however wasn't dead and then it tried to chase them again. When Jamie got near the bottom of the mountain he told Professor Travers where to find it. The Monks were going to kill the Doctor until he brought a long lost artifact back to the monks. The intelligence wanted the Doctor alive. Meanwhil Jamie and the monks set a trap to capture a Yeti. The trap worked as the Yeti fell to the ground. The other Yeti left them. This is when the Doctor discovers that the Yeti are actually robots and that its control sphere was missing. The sphere was moving and it found the Yeti. The Great Intelligence almost succeed with its plan but the Doctor along with Professor Travers destroyed the Yeti and the Intelligence.
The Doctor met the Yeti again in the London Underground Station in the 1960s. It turns out that the Great Intelligence isn't actually dead and that again it wants to take over the world. Professor Travers brought a Yeti back from Tibet and told his daughter that he fixed the sphere but it had vanished. The sphere broke through a window in a museum where the Yeti was and it reactivated the Yeti and changed its appearence. The Yeti killed the head of the museum. The Doctor, Jamie and Victoria were trapped in the Tardis which was being surrounded by lots of web. After a while they were released and they explored the quiet London. They saw soldiers who were from UNIT ready to blow up the tunnels. The Doctor ran off to investigate. Jamie and Victoria followed the soldiers until they were caught. The Doctor hid under the railway tracks and saw two Yeti soldiers with web guns firing at the explosives and covering them with this web that covered the Tardis. The Doctor looks at the explosives which don't detinate due to the web. When the Doctor finds Jamie and Victoria the invasion force has begun with the streets of London full of Yeti robots. The Doctor gets help from a colonel Lethbridge-Stewart and his men to try and defeat the Yeti. The Doctor along with Professor Travers who really was an old man by now helped to destroy the Great Intelligence once and for all leaving all the Yeti robots to deactivate.
The Doctor and Brigadier Lethbridge-Stewart met the Yeti again but in the Death Zone on Gallifrey. The Doctor assumed that the Yeti was left there from the previous Games of Rassilon. The Doctor and the Brigadier went into a cave which would take them to the tomb of Rassilon. But they were followed by a real live Yeti. The Doctor and the Brigadier hid in a small space in the cave where the Yeti couldn't get in. The Doctor however sealed him and the Brigadier in the cave and managed to drive the Yeti away from them.
The Yeti have appeared in:
Season 5: The Abominable Snowmen and The Web of Fear
Doctor Who's 20th Anniversary: The Five Doctors