Called the 'King of Fruit', durian are large (40cm long) hard spiky fruit with a pulply flesh inside. They are supposed to have a stink and you'll sometimes see 'No Durian' signs in the hotels, but that's something of an exaggeration. The smell is very mild when compared to old fish sauce for example and is reminiscent of, well flatulence.
You split the outer case with a large cleaver, break it open and inside in pockets you'll find a soft pulpy flesh wrapped around large stones (shown below), this is the part you eat. The flavour is complex with some sweet and some bitter notes.