The DC and effect depend on the task you attempt.
First Aid: You usually use first aid to save a dying character. If a character has negative hit points and is losing hit points (at the rate of 1 per round, 1 per hour, or 1 per day), you can make him or her stable. A stable character regains no hit points but stops losing them.
Long-Term Care: Providing long-term care means treating a wounded person for a day or more. If your Heal check is successful, the patient recovers hit points or ability score points (lost to ability damage) at twice the normal rate: 2 hit points per level for a full 8 hours of rest in a day, or 4 hit points per level for each full day of complete rest; 2 ability score points for a full 8 hours of rest in a day, or 4 ability score points for each full day of complete rest.
You can tend as many as six patients at a time. You need a few items and supplies (bandages, salves, and so on) that are easy to come by in settled lands. Giving long-term care counts as light activity for the healer. You cannot give long-term care to yourself.
Treat Wound from Caltrop, Spike Growth, or Spike Stones: A creature wounded by stepping on a caltrop moves at one-half normal speed. A successful Heal check removes this movement penalty.
A creature wounded by a spike growth or spike stones spell must succeed on a Reflex save or take injuries that reduce his speed by one-third. Another character can remove this penalty by taking 10 minutes to dress the victim's injuries and succeeding on a Heal check against the spell's save DC.
Treat Poison: To treat poison means to tend a single character who has been poisoned and who is going to take more damage from the poison (or suffer some other effect). Every time the poisoned character makes a saving throw against the poison, you make a Heal check. The poisoned character uses your check result or his or her saving throw, whichever is higher.
Treat Disease: To treat a disease means to tend a single diseased character. Every time he or she makes a saving throw against disease effects, you make a Heal check. The diseased character uses your check result or his or her saving throw, whichever is higher.
Action: Providing first aid, treating a wound, or treating poison is a standard action. Treating a disease or tending a creature wounded by a spike growth or spike stones spell takes 10 minutes of work. Providing long-term care requires 8 hours of light activity.
Try Again: Varies. Generally speaking, you can't try a Heal check again without proof of the original check's failure. You can always retry a check to provide first aid, assuming the target of the previous attempt is still alive.
Special: A character with the Self-Sufficient feat gets a +2 bonus on Heal checks.
A healer's kit gives you a +2 circumstance bonus on Heal checks.
The character can greatly speed a patient's recovery of hit points.
Quicken Recovery: The character can allow a character to regain hit points in a single hour as if he or she had provided long-term care for a full day (2 or 3 hit points per level, based on activity). The character can quicken the recovery of up to six patients at a time. No character's recovery can be quickened more than once per day (even by different healers).
Perfect Recovery: The character can allow a character to regain hit points in a single hour as if he or she had provided long-term care for a full week (2 or 3 hit points per level per day, based on activity). The character can use perfect recovery on up to six patients at a time. No character's recovery can be perfected more than once per day, nor can perfect recovery and quicken recovery both be used on the same patient in the same day (even by different healers).
Source: SRD 3.5 Combined Skills