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.
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.
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.
The Daleks later fought Mechanoids on Hesperus.
On Lethe, Davros combined organic tissue with Mechanoid shells to create 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. Mechanoids do not seem to be evil nor good they are only violent when their home planet is threatened otherwise they seem to be very peacful and open to visitors Ex: When The Doctor along with his companions encountered a Mechanoid it(or he) asked them to follow it(or him) back to the city why the Mechanoids held them there was they were possibly curious about The Doctor and his companions as for Steven since he crashed onto the planet the Mechanoids may have feared it was a terroist bombing thus,accusing Steven as a terrorist agent trying to attack their city.