Life force, also known as mutagenic revivification enhancement energy, (PROSE: The Deviant Strain) was once described by the Eleventh Doctor as "basically the 'soul' of a living being". (TV: The Doctor, the Widow and the Wardrobe)
Many creatures had the ability to drain another's life force like Fendahl, (PROSE: The Left-Handed Hummingbird) or the mutated Lazarus, which could leave the victim looking like a dried husk. (TV: The Lazarus Experiment) Some spacecrafts could also use life force as an energy source. (PROSE: The Deviant Strain)
The First Doctor's life-force was drained by Mondas during the Cybermen's attack on Earth in 1986. This drain on his vitality triggered the Doctor's regeneration into his second incarnation. (TV: The Tenth Planet)
When Jo Grant was accidentally sent to an alternate Earth, her arrival caused the life force of her alternate counterpart native to that universe to disappear, causing her death. The Third Doctor managed to break through into this universe to retrieve Jo, an act that caused his own counterpart to die in the same way. (COMIC: Dead on Arrival)
Abaddon fed off the life-force of living creatures, completely draining it from them. Jack Harkness, being a fixed point in time, had an over abundance of life-force. When Abaddon attempted to steal Jack's, there was too much, and Abaddon was destroyed. (TV: End of Days)
After his botched resurrection by the Disciples of Saxon, the Saxon Master was able to manipulate his life force. At the cost of slowly burning it out, he was able to jump mind-boggling distances, nearly flying, and to manipulate electricity-like bolts which he could fire from the palms of his hands. (TV: The End of Time) After his stay on the saved Gallifrey during which the Time Lords cured his "little condition", the Master was restored to normal, no longer dying or suffering of a ravenous hunger, but stripped of his special abilities. (TV: The Doctor Falls)