The Phoenix was a magnificent, reddish-black firebird that was larger than an eagle, with a saffron chest, its head crested with scarlet, and its tail purple.
The Friends of Narnia encountered a Phoenix in a garden in Aslan's Country after Aslan ended the Narnian world. It roosted in the tree under which sat the thrones of Frank I and Helen, the first king and queen of Narnia.
Shortly after the creation of Narnia, Digory Kirke encountered a similar bird roosting in the Western Wild, in the Garden of Youth, where Aslan sent him to retrieve an apple. Although this bird is never called a Phoenix, both bird and garden are very similar to the Phoenix and garden in Aslan's country, so it is possible that they are in fact the same bird.
During the battle, the Phoenix was used to create a wall of fire; it was signaled by the launching of an arrow, which exploded. Jadis's black-bearded Dwarf archers tried to shoot the bird down, but it dodged their arrows.
The Phoenix was successful in creating the wall of fire, though it did not hold off the White Witch's Army for long, as Jadis used her magic to clear the fire from her path.
In the film, the Phoenix appears to have a forked tail, and can set itself on fire at will.