Units have power, which is a measure of their ability to inflict damage and sustain damage. Power is most often shown as a single number in the top right of the card. For example, has a power of 3. When it strikes, it will inflict 3 damage. If it takes 3 or more damage in a turn, it will die.
Some minions have split power. Units may not have less than zero power.
See the "zero" codex entry for more details on units with zero power.