The Luggage is a large chest that follows Rincewind literally wherever he goes – even onto Roundworld, which Rincewind initially only visited virtually. It is made of sapient pearwood (a magical, intelligent plant which is nearly extinct, impervious to magic, and only grows in a few places outside the Agatean Empire, generally on sites of very old magic). It can produce hundreds of little legs protruding from its underside and can move very fast if the need arises. It has been described as "half suitcase, half homicidal maniac"
Its function is to act as both a luggage carrier and bodyguard for its owner, against whom no threatening motion should be made. The Luggage is fiercely defensive of its owner, and is generally homicidal in nature, killing or eating several people and monsters and destroying various ships, walls, doors, geographic features, and other obstacles throughout the series. Its mouth contains "lots of big square teeth, white as sycamore, and a pulsating tongue, red as mahogany." The inside area of The Luggage does not appear to be constrained by its external dimensions, and contains many conveniences: even when it has just devoured a monster, the next time it opens the owner will find his underwear, neatly pressed and smelling slightly of lavender. It is unknown exactly what happens to anyone it 'eats'.
One of the most notable features of The Luggage is its ability to follow its current owner anywhere, including such places as inside its owner's mind, off the edge of the Disc, Death's Domain, inside the Octavo, the Dungeon Dimensions, and even (literally) to Hell and back. Like all luggage, it's constantly getting lost and having to track its owner down. It has only one way of overcoming obstacles, and that is by simply ignoring them and smashing a hole through them — including a wall to a magic shop that had since relocated to another city by magical means.
The Luggage first appears as the property of Twoflower, the Disc's first tourist in The Colour of Magic (at a bar no less, where all the patrons decide to stare at their drinks after witnessing the arrival of the Luggage). When Twoflower returns home in The Light Fantastic he gives the luggage to Rincewind, and it follows him through several sequels. Twoflower says he got it by asking for "a travelling trunk" at one of about a dozen magical shops – which are not limited by the constraints of time and space, to their owners' dismay – which is exactly what he got. According to "The Light Fantastic", this chain of stores was born when an impatient sorcerer was served rather poorly. When Rincewind eventually visits the Counterweight Continent, Twoflower's home, he finds many items similar to Luggage travelling with their masters.
Pratchett says (at the beginning of Sourcery) that he got the idea for the Luggage when he saw a tartan suitcase with dozens of little wheels moving as though it had a mind of its own while an American tourist pulled it along.
Source: Wikipedia