Ravenclaw is one of the four Houses of Hogwarts School of Witchcraft and Wizardry, founded by Rowena Ravenclaw. Members of this house are characterised by their wit, learning, and wisdom. Its house colours are blue and silver, and its symbol is a raven. The house ghost, who in life was the daughter of the house's founder Rowena Ravenclaw, is the Grey Lady. Ravenclaw roughly corresponds with the element of air; the House colours blue and bronze were chosen to represent the sky and eagle feathers respectively, both having much to do with air.
Ravenclaw House prizes learning, wisdom, wit, and intellect in its members. Thus, many Ravenclaws tend to be academically motivated and talented students. Ravenclaws also pride themselves in being original in their ideas, and methods. It's not unusual to find Ravenclaw students practicing especially different types of magic that other houses might shun. Hermione Granger, an extremely intelligent witch and the top student in her year, was sorted into Gryffindor, though she admitted that the Sorting Hat had seriously considered placing her in Ravenclaw. Students in Ravenclaw can also be quirky and possess unusual intellectual interests. Ravenclaws generally accept and celebrate these eccentrics.
According to Slytherin prefect Gemma Farley Ravenclaws are so competitive when it comes to academic success that they are known to backstab each other, and likely other students, in order to get the top marks, while Hufflepuff prefect Gabriel Truman noted that they are so proud of the success of famous members, such as Laverne de Montmorency, that they claim any intelligent wizard as a member of Ravenclaw, such as Bridget Wenlock, who was in fact a Hufflepuff.
Ravenclaw House appears to have little rivalry with the other houses, except in Quidditch (in which case it is most fiercely competitive with Hufflepuff). Harry Potter believed that they tended to side with Gryffindor in its rivalry with Slytherin, as did Hufflepuff house. However, during the 1994 Triwizard Tournament, Ravenclaws supported Hufflepuff Champion Cedric Diggory over Harry. Notably, many Ravenclaws joined Dumbledore's Army and fought in the Battle of Hogwarts along with Gryffindors and Hufflepuffs.
The Ravenclaw common room is in one of the castle's towers and is wide and circular. You have to climb a tight spiral staircase to get there. It has graceful arched windows, and the walls are hung with blue and bronze silks. The domed ceiling is painted with stars, which are echoed in the midnight-blue carpet. Tables, chairs, and bookcases cover the expanse of the floor, and a white-marble statue of Rowena Ravenclaw sits next to the door that leads to the dormitories above. Harry Potter entered the common room in 1998 while searching for clues to the location of one of Voldemort's Horcruxes.
Unlike the other common rooms in the school, a logical riddle given by a bronze eagle-shaped knocker must be answered to enter. This could prove troublesome as anyone with enough intellect could intrude into the dormitory, as is shown in 1998 where Professor McGonagall had no trouble entering by answering the riddle correctly (though she was a "hat stall" between Gryffindor and Ravenclaw). However, Hermione stated in her first year when figuring out the complicated riddle, beneath the trapdoor, that most wizards and witches do not have much sense of logic, which might be something that Ravenclaw noticed herself and decided to use clever riddles instead of passwords, as they are safer than some words.
The head of house before and during Harry Potter's time was Filius Flitwick. It is unknown when he became head of house, but it is known that he was teaching Charms in 1971. It is also unknown who he succeeded.