In the ShowEdit
The Time Lords first appeared in the War Games, where they put the Second Doctor on trial and ultimately made him regenerate. After this, they became slightly recurring characters, other than the Doctor and the Master, of course.
They had 2 hearts and a binary vascular system, allowing them to breathe in space for longer periods of time. They also had the ability to regenerate.
In the ModEdit
The Time Lords have appeared in the mod since November 2013, in what is called the 50th Anniversary Update. In the mod, their name is spelt 'Timelord' instead of 'Time Lord'. They are also hostile, though are planned to be changed to a neutral mob in the future. The Time Lords will also be able to regenerate in future updates.