Anyone and anything that they possessed are called Controllers.
While not possessing someone, Yeerks' normal form are greatly resembles large slugs with 4 fin-like limbs and greenish gray skin. They lacks eyes, though possesses optical and sensory tentacles on their head. However, once they enter a host's ear and brain, their body shifts into near-liquid form so they could pass through orifices within ear canal and possesses the host's brain, effectively control their body and mind.
Once they sought for world domination after stole various resources from Andalites, they formed hierarchy among themselves, which are:
Council of Thirteen
The Council of Thirteen is at the top of the Yeerk hierarchy. One of the members of the Council is the Emperor, but only the Council members know which one. There are a variety of hosts on the Council including Hork-Bajir and Taxxons. There are no humans on the Council, and no Andalites as Visser Three/One was never appointed to the Council.
Just below the Council of Thirteen are the Vissers, generals of the Yeerk empire. The lower the number the higher ranking the Yeerk, therefore Visser One is the highest, and this goes on down until Visser 47 or so.
Known Vissers: Esplin 9466
Below Vissers are Sub-Vissers. They are the colonels of the Yeerk empire, having subordinates but still taking orders from a superior officer. The same applies to Sub-vissers as Vissers, the lower the number the higher the ranking.
The lowest ranking members of the Yeerk empire are the standard controllers. However, there are high ranking controllers such as Chapman, and there are low ranking controllers such as Tom. Serves Yeerk Empire as soldiers, workers, spies, etc.
Notable Controllers: Esplin 9466 Lesser