The triptych as a whole shows a narrative sequence that starts on the left with the creation of Eve, continues in the centre with the eponymous Garden of Earthly Delights, and ends up on the right in Hell.
Man starts in Paradise, falls because of Eve's disobedience when she eats the apple, and ends up condemned to eternal damnation in retribution for his sins.
Bosch's Hell is a more richly realised and individual Hell than the normal medieval underworld. It's a world of weirdness. In the background, cities burn; the scene is cold, with a blueish tinge to many of the colours, and it's full of intense detail, and very strange detail at that.
Bosch's nightmare vision is like Breughel on steroids - the same feeling of a world full of people all busy pursuing their own activities, and full of precisely observed everyday objects, but in Bosch's world, everything has been strangely transformed into the stuff of nightmares.
There's a knife which has ears, self-playing pink bagpipes, and a big pair of buttocks being used as a hymn book by the choir.
As this is Hell, people are being tortured, but Bosch invents new ways of processing human flesh. A bird-headed demon in a high chair swallows and excretes human bodies, a parody of God creating man.
There are people being impaled, one on an oboe - musical references are everywhere in this painting, from the bagpipes to a big hurdy-gurdy, as if hell is made up of noise. Perhaps Bosch thought music was one of the biggest temptations to sin, or perhaps he just didn't like it.
At the centre of the painting is perhaps its greatest oddity, a hollow man whose body is made out of, but is also pierced by, a strange tree. Some art historians think the face, turned towards the viewer, is a self-portrait; that can't be proved, but the face seems to be questioning. Inside the man's eggshell-like broken body, three tiny figures are sitting at a dinner table.
The meaning of this strange picture will continue to be debated as long as there are art historians, but its emotional power is undeniable.