See augment for how to increase the base value of your hero's stats.
Life (a.k.a Health, Hit Points) is a measure of the damage the hero can absorb. It is affected by Armor and Outfits.
Strength increases attack damage of the hero (given as a min/max pair of numbers). It is affected by Weapons and Outfits.
IntelligenceIntelligence increases skill damage (every 100 intelligence gives hero skills 1% effect). It is affected by Helmets and Outfits.
Agility increases attack speed. It is affected by Amulets. By running some experiments, it has been found that each 200 agility increases a hero's attack speed by 5%. (thanks to soltys13 and popington, on AG).
Each 40 agility increases a hero's attack speed by 1%
Armor reduces physical damage taken (except magic damage), and also Sir Lance Alotte's Thorn efficiency. Can be affected by skills, talents and castle Technologies, but not by augmenting or equipment.
Every 100 points increases 1% armor.
Low: 1% - 20%
Medium: 21% - 50%
Heavy: 51% - 80%
Beefy: over 80%
Barracks soldiers have the following base armor values:
Other upgrades (Gladiators, Clergymen) and Heroes have no base armor.