The immediate cause is a contraction of the diaphragm. This causes you to take in air very quickly, and then when the glottis closes (the place between the vocal chords) the air intake is suddenly stopped. It is this stoppage of air that creates the hiccup sound and the uncomfortable, breathless sensation that goes with it.

The diaphragm can tighten up for no apparent reason, but there are some causes that make hiccups more likely. The one that most of us probably meet at some time is eating or drinking too quickly, which makes the stomach bloat and interferes with the flow of air. Especially, alcohol drunk quickly can have this effect. You can also get hiccups if it suddenly gets much hotter or colder, as this can also cause the diaphragm to contract, or if you get a shock (this may be the reason why some people think you can 'cure' hiccups by giving someone another shock.)

If hiccups recur or go on for a very long time, there may be underlying stomach or other problems, and you should see a doctor.