The Narnian sun was a large, bright object in the Narnian sky. It existed from the moment that Narnia first came into existence, till the day its history came to an end.
Little is known about the sun's composition, but it's possible that it was not entirely gaseous, as there was talk of Mountains and Valleys of the Sun. Fire-Flowers and fire-berries grew in these valleys, and had magical healing properties. Fire-Flowers were used to make Lucy's Cordial, and fire-berries could make a star younger. (It's possible that the names of these places were just that, mere names, and they were not really part of the actual sun itself, but there's no way to tell.)
The colour of the sun behaved in a similar way to the world of Charn's sun. For most of its life, the sun was yellow, but towards the end of its life, it grew larger and redder.
The sun was created at the beginning of Narnia, on the day of its birth, by Aslan singing his most magnificent and glorious sound at the time. The sky changed from black to grey, to white, to pink, and to gold, as the sun was created, which then rose in the east. The stars and landscape of Narnia had already existed at the time.
As the sun rose, it appeared younger than Earth's sun. Narnia's sun gave the impression that it could laugh for joy, as it came up.
At the end of the world, the sun rose for the last time with a dark red colour, and a 20-fold increase in size. It then coalesced with the moon to form a larger ball, like burning coal. This state was unstable, as lumps of sun began to fall down to Earth. The heat of these lumps caused steam to rise from the sea, as they landed.
The sun was then destroyed by Father Time. On the orders of Aslan, he destroyed the sun by squeezing it in his hand, in a similar way to how an orange could be squeezed. This instantly caused the world to turn completely cold and dark. The world ended soon after.
The sun is mentioned in a Calormene expression, "Every morning the sun is darkened in my eyes", which is used to show an ongoing, important frustration.