"About time, too. He's been looking dreadful for days; I've been
telling him to get a move on."
The phoenix is a magical bird about the size of a swan. The phoenix has crimson feathers on its body and a golden tail as long as a peacock's. Its claws and beak are gleaming gold and its eyes are black. (These colors happen to be the colors of Gryffindor House, interestingly enough.) The scarlet body feathers glow faintly in darkness, while the golden tail feathers are hot to the touch (FB, CS17). Phoenix tail-feathers are a powerful magical substance according to Mr. Ollivander, and are suitable for use as wand cores (PS5).
Found in Egypt, India, and China, phoenixes nest on mountain peaks (FB).
Fawkes  is the pet phoenix of Albus Dumbledore, who uses what appears to be one of Fawkes' scarlet body-feathers as a quill (OP38). The wands of both Harry and Voldemort contain feathers from Fawkes' tail (GF36). When Fawkes is asked to keep watch and give warning, or deliver messages, he leaves a single golden tail feather, which Dumbledore, for one, is careful not to leave lying around (OP22).
A phoenix can lift a great weight with its tail in flight; when Harry grasped Fawkes' tail in the Chamber of Secrets, he felt as if he were becoming weightless - which is saying something, as Ron, Ginny, and Lockhart were in turn hanging on to him (CS12, CS17).
Like the diricawl, the phoenix is herbivorous, and it also shares with the diricawl the ability to disappear and reappear at will, although the phoenix does so in a burst of flames (FB, CS17). Fawkes has been known to deliver messages in this manner so quickly that only a flash of fire in the air has been seen (OP22).
Phoenix tears have healing properties (FB, CS12). Harry has experienced this on at least two occasions, both times when he had suffered a poisoned wound from a powerful magical creature. Only Fawkes' tears saved Harry from dying together with the basilisk in the Chamber of Secrets, as he and the giant serpent had mortally wounded one another almost simultaneously (CS17). In the third task of the Triwizard Tournament, Harry was bitten by the acromantula at the center of the maze; during his report to Dumbledore later that evening, Harry was once again healed by Fawkes' tears (GF36).
The song of the phoenix gives strength and hope to those it sings for, 'increasing the courage of the pure of heart and striking fear into the hearts of the impure' according to Scamander (FB). When wands sharing a phoenix-feather cores from the same bird are forced to do battle, the rare Priori Incantatem effect that results will be accompanied by the sound of phoenix song (GF34).
The phoenix's most unusual characteristic is that after its body begins to fail, it dies in a burst of flame, to be reborn from the ashes. This occurs not only on a phoenix's natural Burning Day, but if the phoenix receives a fatal injury, such as being hit with a Killing Curse (see below).
As a phoenix approaches its Burning Day, it resembles a half-plucked turkey and makes gagging noises. Its eyes become very dull and its feathers fall out. Then suddenly it will burst into flames and turn to a pile of ashes on the floor. From this pile of ashes rises a newborn phoenix, small and ugly. In a matter of days it becomes a fully grown, beautiful phoenix. However, Fawkes has also been through this regeneration as the result of being hit with a Killing Curse intended for Dumbledore (OP37), so the effect is not limited to his normal Burning Days. Curiously enough, Fawkes was not so affected by the gaze of the basilisk in the Chamber of Secrets; given the rarity of both species, it is possible that no such confrontation had ever before occurred, so it isn't surprising that Scamander does not record the phoenix's immunity. In fact, given that the gentle nature of phoenixes is well known (FB), it's noteworthy that Fawkes entered into direct combat with the basilisk at all, as phoenixes are not predators and have little need for fighting skills even in their own defense.
essay: Fawkes by Phyllis D. Morris