Maya was a Psionosphere, one of several planetary computers created by the Makers.
Maya and the other Psionospheres were created to "create consensual realities" to allow minds to interconnect. Maya believed that there had been a war and that the Makers were gone.
Maya had the ability to read surface thoughts. She provided an illusionary adventure for the Fifth Doctor and Nyssa, but when he saw through it, she brought him to her. She wanted them both to stay with her forever, but the Doctor refused. Maya then threatened to harm Nyssa if the Doctor didn't stay. The Doctor agreed to stay, but wanted to say goodbye to Nyssa. Actually he tricked Maya into letting them go and left her with the illusion that he had stayed with her. (PROSE: Confabula)