What does "life force" mean? The term life force is used a number of times during the series. Does life force simply mean chakra? If so, then do non-ninjas (such as Teuchi, the owner of Ichiraku Ramen) not have life force? Is there a canonical definition for this term?
The way I see it, Life Force is a completely different energy from chakra.


Chakra can be depleted and restored (even though if you deplete it too much, you die). When Life Force gets used in a technique, the user's vitality usually takes a hit, as well as his life-span. (Nagato was left completely drained, and never recovered from using the Gedo Mazo in battle).
Life Force was seen to be transferred and infused in other individuals to revive them. Both "super healing" and actual bringing back from the dead was seen. Mere "chakra" does not have this property.
Life Force seem to have some sort of connection with the Yang (or physical) energy your body produces, and mixed with Yin to form chakra. 


Spoilers:


   Not to mention humans once lived without chakra, and it was given to them by the God Tree, the term "Life Force" indicates that without it there can be no life, hence the two are different.


So what is Life Force?

The term is very hazy, it wasn't explicitly explained or discussed in the series, but from what I get, it's a completely different energy, closely related to the Yang energy, and it determines your vitality and life-span.