Positive Energy is a "damage" type used by most healing spells. It's the essence of life, and exposing most living things to it instantly heals damage, cures ability damage and negative levels or even reverses death.
Positive energy's opposite is Negative Energy. When the two types meet, they cancel out, destroying both. Unlike most life, Undead are instead fueled by Negative Energy. Positive Energy can damage, stun, fear or destroy most undead. Note that Pale Master enhancements wizards using a shroud of undeath are not harmed by healing spells, although some damage-only positive energy effects such as Disrupt Undead can still hurt them. Negative spells should be used to heal undead. Typically, undead players or players with undead minions can do this themselves.
Constructs and Inanimate Objects are unaffected by either energy type. Living Constructs, such as Warforged are 50% resistant to Positive Energy. Repair spells can be used to effectively heal these types of creature.