A spasm of the diaphragm (the diaphragm is a sheet of muscle which separates the heart and lungs from the lower organs of the abdomen) which causes a sudden expansion of the lungs, intake of breath, and the epiglottis slams shut. The main theory about why is that the body is short of oxygen and needs a quick boost.
Try breathing in a brown paper bag for a few minutes. This is supposed to increase the CO2 in your system making the medulla oblongata alter the requirement for oxygen thereby dehiccupifying the clack valve confunctivator. Or something. Anyway, it always works for me.
I thought the reason of hiccups was the body thinking that there is a bit of air in the stomack, and trying to get rid of it... ( I sometimes get a single hiccup spasm followed by a burp )
Hiccups are caused by kebabs which prevent the Co2 gas of previously consumed alcoholic drinks escaping upwards through the body. An easy and effective cure is to (which is easy after a few gassy beers...) induce vomiting : this will hopefully unblock the gass pipe. May I recommend a few rennies or other indegestion remedies (especially when drinking cider) to prevent hiccups during drinking periods. However, a high concentration of both acid and alkali (eg. rennies and cider) will later produce more flatulence than normally expected - not a problem unless you're married or a admirer of hot and spicey food : like a kebab with chilli sauce after a few ciders.