Life is the amount of health/hitpoints that a player character has. Each time a player receives damage, that amount may go through some transformation based on skills, gear, stats, or resistances before it is then subtracted from their hitpoints. When it reaches zero, the player dies. The player's life is represented by a red orb at the bottom left corner of the UI.
Diablo III[]
In Diablo III, the amount of hitpoints that a player character has is based on the total Vitality.
Players can recover life by using healing skills, potions, shrines, gathering Health Globes, or talking to the town healer.
Diablo II[]
Life can be recovered with potions of healing, speaking to a healing NPC, or with the Life Stealing weapon modifier. Life can also be replenished by the use of Healing Potions and Rejuvenation Potions. Healing potions heal over time while Rejuvenation potions act instantly.
Healing NPC's[]
The amount of maximum life can be affected by items with life bonuses, or Stat Points invested in Vitality. Maximum life can also be increased by certain class skills or auras such as the Druid's Oak Sage.
Life can also be increased permanently by use of certain elixirs prepared by talented alchemists like Alkor in Kurast. In Diablo II, it increased life permanently by 20.
Note that you cannot die from poison, because poison can only lower your hit points to 1. But your Hireling can die from poison.