The Order of the Phoenix is a fictional organization created by JK Rowling for her series of Harry Potter. It consists of a group of wizards who band together to fight against Lord Voldemort and his Death Eaters. Within the framework of the books, the Order of the Phoenix is a good organization, while the Death Eaters are considered evil. The fifth book in the series is named for the Order, and the Order, along with its members, plays a pivotal role in the series.
Albus Dumbledore initially founded the Order of the Phoenix in the 1970s, when Voldemort was first rising to power. He gathered a crew of talented witches and wizards together to oppose Voldemort and his followers. The Order was named for Fawkes, Dumbledore's pet phoenix, who also happens to be an important character in the books. During the 1970s, many members of the Order of the Phoenix died or vanished while opposing Voldemort. Among the dead are Lily and James Potter, Harry Potter's parents.
When Voldemort rises again in Harry Potter and the Goblet of Fire, the fourth book, Dumbledore quickly reforms the order, despite claims by the Ministry of Magic that Voldemort has not reappeared. The later half of the series documents the battles fought by members of the Order of the Phoenix, several of whom die or are injured. Some of the members are old members from the 1970s, while others are newly joined. The combined forces of the Order of the Phoenix represent formidable skill, along with a desire to conquer evil in the world.
In the fifth book, the headquarters of the Order are established as being at Number 12, Grimmauld place. Members can only find out the location of the headquarters from Albus Dumbledore, who acts as Secret-Keeper for the order. A Secret-Keeper is an individual used to hold an important secret such as the location of a witch or wizard who may be in danger. The secret is embedded within the Secret-Keeper using the Fidelius Charm, which is said to be a very complex piece of magic.
Readers learn about the Order of the Phoenix in the fifth book, when the series turns much darker with the return of Voldemort. In order to join, wizards must be 17 and out of school. Membership in the Order of the Phoenix can prove to be very dangerous, as Voldemort and the Death Eaters will stop at nothing to achieve their ends.