Life is a multiplier on the number of hit points your character has, aside from a starting 15 hit points all characters have. See the HP page for further details on the formula. 100 Life is the standard value for most races. Liches have 80 Life and Fairies have 40 Life. This means that, without adding to their life scores, fairies will have less than half the HP of other races. The minimum of 15 HP exists specifically to help keep races like fairies and metals from dying from their low Life scores too easily. A Golem has 150 Life, adding up to 3.75 times as much HP over the starting 15 HP, even before you consider the bonuses to Constitution they have.
The Ascetic feat will increase Life by 5 for every feat level, up to five levels. The attributes on some items may increase or decrease Life. Equipment made of rubynus will increase Life by 3. Hero cheese will permanently increase life by 3. Only Hero Cheese can theoretically be used to gain infinite life scores, (although all attributes are capped at 2000,) although the extreme number of Little Sisters you would have to catch would rise to prohibitive levels before you could get terribly far.