Although mechanically similar to life, it is flavorwise not the same. Flavorwise, damage that would otherwise be fatal doesn't "kill" a planeswalker, it just discourages them from helping you and causes them to leave (sent to graveyard).
- 1. Part of a card that only planeswalkers have. A planeswalker card’s loyalty is printed in its lower right corner. See rule 209, “Loyalty.”
- 2. A characteristic that only planeswalkers have. See rule 306.5.
- 209. Loyalty
- 209.1. Each planeswalker card has a loyalty number printed in its lower right corner. This indicates its loyalty while it’s not on the battlefield, and it also indicates that the planeswalker enters the battlefield with that many loyalty counters on it.
- 209.2. An activated ability with a loyalty symbol in its cost is a loyalty ability. Loyalty abilities follow special rules: A player may activate a loyalty ability of a permanent they control any time they have priority and the stack is empty during a main phase of their turn, but only if none of that permanent’s loyalty abilities have been activated that turn. See rule 606, “Loyalty Abilities.”
- 107.7. Each activated ability of a planeswalker has a loyalty symbol in its cost. Positive loyalty symbols point upward and feature a plus sign followed by a number. Negative loyalty symbols point downward and feature a minus sign followed by a number or an X. Neutral loyalty symbols don’t point in either direction and feature a 0. [+N] means “Put N loyalty counters on this permanent,” [-N] means “Remove N loyalty counters from this permanent,” and  means “Put zero loyalty counters on this permanent.” Loyalty symbols may also appear in abilities that modify loyalty costs.
- 120.3c Damage dealt to a planeswalker causes that many loyalty counters to be removed from that planeswalker.
- 704.5i If a planeswalker has loyalty 0, it’s put into its owner’s graveyard.
- Damage dealt to a planeswalker causes that many loyalty counters to be removed from it. This is a damage result; see rule 119.3c.
- A planeswalker with loyalty 0 is put into its owner's graveyard. This is a state-based effect; see rule 704.5i.
- Mark Rosewater (November 05, 2007). "Planeswalk on the Wild Side, Part I". magicthegathering.com. Wizards of the Coast.
- Doug Beyer (August 26, 2009). "Your Mailbox is Over Vorthosity". magicthegathering.com. Wizards of the Coast.
- Doug Beyer (November 25, 2009). "Brooms, Planes, and Valakuts: Tales from the Inbox". magicthegathering.com. Wizards of the Coast.