There are numerous ways to restore lost HP.
The most mundane is by resting and allowing time to do the job.
For each day of rest, 1 hit point of damage is restored.
After 30 game days have passed, HP accrue at the rate of 5 per day thereafter.
The "laying on of hands" by paladins, spells, potions, and various magical devices will quickly restore many lost HP.
A wish spell can be used to restore lost hit points to several characters at once.
Hit points can never exceed the total rolled for the character, plus bonuses.
 

Q. The Players Handbook and the
Dungeon Masters Guide give 2
different rates for character healing.
Which is correct?
A. This is one of those points in the roolz
where the publication date defines
what is right or wrong.  As a general
principle, a later edition/printing
date on a particular rool book over-roolz
an earlier rool book if there is
any difference. Therefore, the healing
rates given on p82 of the DMG (in
the section Recovery of HP)
are the correct ones.
(Imagine #18)