Text quoted from The New Discworld Companion.

Death. The Defeater of Empires, the Swallower of Oceans, the Thief of Years, the Ultimate Reality, the Harvester of Mankind, the Assassin against Whom No Lock Will Hold, the only friend of the poor and the best doctor for the mortally wounded. An antropomorphic personification. Almost the oldest creature in the universe (obviously someone had to die first...).
     He is a 7-foot-tall skeleton of polished bone, in whose eye sockets there are tiny points of light (usually blue). He normally wears a robe apparently woven of absolute darkness - and sometimes also a riding cloak fastened with a silver brooch bearing his own personal monogram, the Infinite Omega. He smells, not unpleasantly, of the air in old, forgotten rooms.
     Death's scythe looks normal enough, except for the blade, which is so thin you can see through it - a pale blue shimmer that could slice flame and chop sound. His sword has the same ice-blue, shadow-thin blade, of the extreme thinness necessary to separate body from soul.
     His face, of necessity, is frozen into a calcarous grin. His voice is felt rather than heard. he is seen only by cats, professional practicioners of magic, and those who are about to die or are already dead - although there is some evidence that he can be glimpsed by those in a heightened state of awareness, a not uncommon state given the Discworld's normal alarums. When he needs to communicate with the living (i.e. those who are going to continue living) he is perceived very vaguely by them in some form that does not disturb them. There was a period when he made an effort to appear in whatever form the client expected (scarab beetles, black dragons, and so on). This foundered because it was usually impossible to know what the client was expecting until after they were dead. He decided that, since no one ever really expected to die anyway, he might as well please himself and he henceforth stuck to the familiar black cowled robe.
     His horse, though pale as per traditional specification, is entirely alive and called Binky. Death once tried a skeleton horse after seeing a woodcut of himself on one - Death is easily influenced by that sort of thing - but he had to keep stopping to wire bits back on. The fiery steed that he tried next used to set fire to the stables.
     Despite rumour, he is not cruel. He is just terribly, terribly good at his job. It is said that he doesn't get angry, because anger is an emotion, and for emotion you need glands; however, he does seem to be capable of a piece of intellectual disapproval which has a very similar effect. He is a traditionalist who prides himself on his personal service, and, despite the absence of glands, can become depressed when this is not appreciated.
     Humanity intrigues Death. He is particularly fascinated by mankind's ability to complicate an existence which, from Death's point of view, is momentary. He appears to spend a lot of time rying to learn, by logical deduction, the things that humanity takes for granted. In th eprocess, he seems to have developed what can only be called preferences and likings - for cats, for example, and curry. He has tried to take up the banjo, but lack any skill with such a living thing as music.
     Death has a property not locatable on any normal atlas, on which he has called into being a house and garden. There are no colours there except black, white and shades of grey; Death could use others but fails to see their significance. And, because he almost by definition lacks true creative ability - he can only copy what he has seen - no real time passes in his domain. Now do things live or grow in the normal sense, unless they are brought in from outside, but they exist in an apparently unchanging, healthy state.
     He appears to derive his opinion of how he should live by observing people, but the nuances consistently escape him. He has a bedroom, for example, because although Death never sleeps, it's right that houses have bedrooms. He also has a bathroom, although the ablutionary fixtures were supplied by a plumber from Ankh-Morpork because plumbing is among those activities where Death's constructive abilities find themselves cramped; he was not aware that pipes were hollow inside, for example. On his dressing table he has a pair of silver-backed hairbrushes and a little glass tray for cufflinks, despite having neither hair nor cuffs. He thinks that's what he ought to have.
     As with all creatures that have existence, Death has an hourglass/lifetimer that measures the length of his days. His is several times the size of normal people's glasses, and is black, thin and decorated with a complicated skull-and-bones motif. It has no sand in it.
     There is a strong suggestion in the books that Death is somehow on our side, which is borne out by events in Thief of Time and Reaper Man.