The C'Tan were portrayed in the original Necron Codex as a Xenos race that evolved very early on in the history of the galaxy. They are solar vampires, sun baskers, themselves consisting of continent sized streams of matter that float within suns, feeding on their energies.They were intelligent, but their minds worked in ways totally different to most other races.
Their existence was discovered by the Necrontyr, a more traditional race, who evolved on a planet orbited by a star occupied by a particularly powerful C'Tan, later known as the Deathbringer. The Necrontyr (a highly advanced race) developed means of communication with the C'Tan, and eventually found ways to transfer their gaseous forms to a "necrodermis," a liquid metal shell that contained their energies.
The Necrontyr came to worship the C'Tan as Gods, and with their assistance managed to acheive a number of stunning victories in the "war in heaven" a vast battle with the "Old Ones" an ancient, mysterious and relatively benign race who created the webway.
The C'Tan eventually came to dominate much of the Necrontyr civilisation, and used their obsession with death and immortality to trick them into a kind of species-wide ascension/sublimation into metallic bodies, thus creating the Necrons. To the horror of the more senior members of the Necron civilisation, this process was not all it was cracked up to be. The most senior members of the Necrons found a way to shatter the C'Tan into "shards," in revenge for the effective destruction of their race. The C'Tan are now effectively semi-sentient doomsday weapons used on the battlefield by the Necrons.
This is a pretty much potted history of the C'Tan, but there's a lot we don't know about them. The "shard" concept is a new invention, a product of the latest Necron Codex, which has its supporters and detractors among people who loved the old codex.
As for the Dragon, he was one of 4 C'Tan identified in the original Codex (the others being the Outsider, the Deceiver and the Nightbringer). The second codex identifies many more C'Tan, but the latest Codex suggests (with deliberate ambiguity) that some C'Tan may not have been shattered into shards, and may be out there in the galaxy, plottign away.
For more, I'd recommend reading the Necron codices (they'reboth good, in different ways) and Lexicanum for a summry of the race.
Hope that helps!