The Vist, also called Time Walkers, were creatures able to move across time freely and feed on energy. They had the bodies of greyhounds with tall, giraffe-like legs, small arms, and wizened monkey-faces with two pupils in each eye.
Since they could naturally travel in time, they could not understand the concept of a time machine as an example of "time travelling technology." They could smell time energy. However, their natural ability to travel in time meant that they had a limited understanding of the nature of time travel itself; the Doctor used the analogy that just because seagulls could fly didn't mean that the birds in question could understand the concept of aerodynamics.
They could record telepathic messages for planet-wide broadcasts, and were also able to construct "walls" around different times in the Time Vortex. It occurred to them what would happen if they could fence off sections of time to feed, and then it occurred to them what would happen if they charged other "time-locked" creatures to enter their time. Their base was a colourless/multi-coloured dome that existed outside of time and space. Operating from a shadow Earth, they took control of everything in the universe from 2011 to 2019.