Time Lords appear human, but are different. Time Lords can live for an extremely long time thanks to their ability to regenerate.
Time Lords regenerate after suffering from mortal injury, illness, or old age.
Regeneration repairs and rejuvenates all damage. But a side effect is that it changes the Time Lords physical appearance, and they gain a some what different personality.
Regeneration can be traumatic, and can leave the Time Lord disoriented, and weakened.
"He's regenerating, he's changing"