Mechanoids were large spherical robots originally built by humans in order to help colonize worlds until they were abandoned and forgotten on Mechanus. The Mechanoids became a force in their own right and Mechanus became their home planet, They are also the enemies of the Daleks.
The Mechanoids first encountered the Daleks when they built a space station for refueling on the route to the planet Oric. The Mechanoids used their suspicion ray on one of the Daleks there, causing it to perceive other Daleks as its enemies and kill them. The Mechanoid Interceptor which held the ray was destroyed by the Daleks, but the Dalek ship was then attacked and melted by two other Mechanoid ships. The Mechanoids then broadcast a message to the Daleks on Skaro, warning them to avoid their territory. The Daleks instead began preparing for war with the Mechanoids. (DC: Eve of War)
The Zerovians, seeking to prevent war from breaking out between the Daleks and the Mechanoids, sent their robot agent, 2K, to accomplish this. The robot manipulated the Daleks into destroying the rogue planet Skardal, which they had set on a collision course with Mechanus, then told the Mechanoids that the Daleks had saved them purposely. The Mechanoids then called off the war, although they still considered the Daleks their enemies. (DC: Impasse)
Some Mechanoids were sent by humans to prepare Mechanus for colonization, but Earth got caught up in an interplanetary war and the colonists never arrived. Steven Taylor crashlanded on Mechanus and was taken prisoner by the Mechanoids. The Daleks followed the Doctor's TARDIS to Mechanus and battled the Mechanoids. (DW: The Chase)
The Daleks later fought Mechanoids on Hesperus. (EDA: War of the Daleks)
On Lethe, Davros combined organic tissue with Mechanoid shells to create Juggernauts. (BFA: The Juggernauts)
Dalek Sec was the Commander of a Dalek force whose mission was to wipe out the Mechanoids. Sec commanded from the battlefield, overlooking the field from vantage points. After the Mechanoids were exterminated, Sec returned to the Emperor as ordered. (DWF: Birth of a Legend)
No explanation has been given reconciling the conflicting accounts of the Mechanoids being human servants on television and being an alien race in the comics. One theory is that when the Mechanoids were forgotton on Mechanus by the humans, they became their own race, forgetting their origins and creators.
Behind the scenes
On the DVD commentary for "Day of Armageddon", designer Raymond Cusick contended that the Mechanoids were originally meant to be called the "Mechons". However, the name had to be altered due to the similarity with Dan Dare's main enemy, The Mekon. (BBC DVD: Lost in Time) The Mekon later influenced the visual design of Davros. (BBC DVD: Genesis of the Daleks)