Valar is the name given to the chief Ainur following their descent to Arda. The Ainur were holy spirits created by the supreme deity, Eru Ilúvatar (see Ainulindalë). The most powerful of the Valar was Melkor, who became corrupt, and ceased to follow the will of Ilúvatar. The fourteen remaining Valar continued in Ilúvatar's will.
Everything created by Ilúvatar had both masculine and feminine forms. Thus, there were seven male Valar, and seven female ("Valier"). Some of the Valar were considered siblings, or even spouses, yet these distinctions are difficult to make, as the Valar were ultimately incorporeal beings. Of the fourteen, those eight with the greatest might (called Aratar) were responsible for some attribute of life in Arda (e.g., crafts, mining, agriculture, etc.). The king of the Valar was Manwë.
In addition to the Valar, there were Ainur of lesser might called the Maiar. The Valar ruled the Maiar, who were their students and assistants in governing Arda. The Valar (including Melkor) had the ability to change their physical appearance, or to bear no shape at all. This was also true for some of the Maiar—however, their abilities were not limitless. Some of the Maiar bore the form of animals (e.g., Huan, the hound of the Valar; or the Eagles of Manwë).
History of Composition
Although sequential descriptions of the Valar go back to The Book of Lost Tales, the earliest writing that resembles the Valaquenta is found in the text called Quenta Noldorinwa (published in volume 4 of The History of Middle-earth). It then became Chapter 1 of the Quenta Silmarillion (entitled Of the Valar). In revisions to the Quenta Silmarillion done in 1958, the section was split off into a separately titled work. There is nothing to indicate why Tolkien felt that the piece should stand alone. While it is not a narrative, neither is the chapter Of Beleriand and its Realms, and Tolkien never seems to have considered removing that section.