When a creature receives healing of any kind, hit points regained are added to its current hit points. A creature’s hit points can’t exceed its hit point maximum, so any hit points regained in excess of this number are lost. (97)
Hit points can be restored by:
in limited circumstances, regeneration.
0 hit points and death. If a creature has 0 hits points, it's either dead or on the verge of death. Go here for more details.
Death. A dead creature can’t regain hit points the usual way; special magic is necessary. Go here for details.
Other injuries. Some injuries are not hit-pointed related. Go here for more details.