Rebirth is a druidresurrection ability, the only one of its kind that can be cast while in combat. To distinguish it from the similarly-named [Revive] it is often referred to in game as a combat– or battle–rez, or just "b–rez".
Players that die while affected by a Warlock's Soulstone may revive themselves
Some classes may be able to use abilities to escape combat to then use their resurrection ability. For example a Hunter with Engineering may use Feign Death followed by Goblin Jumper Cables, although since patch 2.3.0 Feign Death no longer removes a hunter from combat during boss fights.
Like all other spells with a cooldown of 10 minutes or more, it cannot be used in Arenas.
In the patch 4.0.1 the cooldown of Rebirth has been increased to its pre-WotLK length of 30 minutes. Moreover druids attempting to use Rebirth in combat sometimes see the “You can no longer resurrect during combat” error message pop up. The mechanics behind this has not yet been explained by Blizzard, and it is not known where the restriction comes from. It is speculated it might be that only one battle rez can be used every 30 minutes in a given group, no matter if other druids have the cooldown free or not. Another possibility is that in any given encounter the spell can only be used once regardless of other druids apart from the spellcaster having the cooldown free.
Due to the cooldown, raids are effectively limited to one battle res per druid per encounter. Because of this limitation, Rebirth use is usually reserved for critical players: tanks, healers or the group's highest-performing DPS
Like Bloodlust/Heroism, groups will often wait until Rebirth is off cooldown before pulling a boss.
Resurrecting a player in combat is inherently dangerous as the target will be dropped in a dangerous environment with very little health. Healers must be prepared to heal very quickly, especially if the target is a tank who needs to pick up aggro quickly. However, if the Druid who casts Rebirth has a Glyph of Rebirth set in his Glyph roster, this danger is effectively removed (since the player who receives the Rebirth will be resurrected with full health in such a scenario).
Similarly, since the target is revived at the caster's location, a druid should be careful not to be standing in a source of damage themselves while casting Rebirth
Rebirth can be used for wipe recovery. If a party is going to wipe and a member of a class with a resurrection spell has already died, the druid can cast Rebirth on that player. The player receiving the Rebirth can wait until after the wipe to accept the resurrection and then continue to resurrect the rest of the party. This is an unconventional use of Rebirth and should only be done with good communication, as it achieves no benefit and wastes the cooldown if the resurrected player accepts before the wipe is complete. Also note that you must rebirth the player outside of the aggro range of any enemies or they will be attacked again after they accept. Make sure the healer you use this tactic with doesn't have any add-ons that auto-accept resurrection spells!
On very rare occasions, the threat attached to restoring X health to a player, along with their life, can cause an aggro switch.
Should you ever feel generous enough to res a corpse found at the side of the road, check to see if it's flagged PvP first!
Edit
2020
Patch 4.0.3 (2010-11-15): The cooldown on this spell has been decreased from 30 minutes back to 10 minutes. 
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Hotfix (2009-04-23): "Nature's Swiftness should properly affect Rebirth." 
Patch 2.3.0 (2007-11-13): The cooldown on this spell has been lowered from 30 minutes down to 20 minutes. 
