The Discworld Creator appears in Eric. He is a little rat-faced man with a put-upon voice made for complaining, who created the Discworld while the main universe was being built, and it was obviously on a budget. It is clear that World Creation is purely a mechanical function and doesn't call for godlike attributes.
There is a legend that tells that the creator passed the recipe of the Pizza to a mortal in a dream, and that he revealed that a pizza was what he intended to do when he created the Discworld, and that what the Creator had in mind was a small cheese and pepperoni affair with a few black olives, but things like mountains and seas got added out of last-minute enthusiasm.
He was not responsible for creating the entire universe, and is somewhat disparaging of it, describing the Big Bang as "showy". After creating the Discworld, he left behind his personal grimoire, the Octavo. This was, apparently, typical absent-mindedness; he says he once created a world and completely forgot the fingles. No-one noticed, because they evolved there and didn't know there should be fingles, but they could tell there was something missing somewhere, and it caused them deep psychological problems.
Rincewind is believed to have had a hand in creating humans on the Discworld, as described in Eric, when he met the Creator he dropped an egg and cress sandwich (with no mayo) that the Creator had brought into being for him into a rockpool, and he believes that this may have kick started evolution, and isn't happy about it (possibly because this was the first good thing to happen to Rincewind, and he wasn't there to see it).
The various aspects of the Creator's act of Creation are remembered vaguely by the spirits of the Octavo, who spend a great deal of time arguing over which event was the true act of Creation. They are described in mythological terms but seem more mundane than they might appear - the "Cosmic Egg" is described as "rubbery", for example -- and it is only later that we learn how mundane these events appeared when the Creator actually performed them (The Cosmic Egg being, for instance, the egg in Rincewind's sandwich).
It is strongly implied that the Creator's physical appearance is a reference to Terry Pratchett himself, and he is a self-parody of Pratchett's own act of creation in writing the novels.