Couscous looks like a grain, but it is actually more like a pasta: granules of crushed semolina flour, bound together with water.
It is mostly made in factories these days: making it from scratch involved rubbing and rolling crushed semolina grains of varying sizes, and spritzing them with sea water so that the particles would adhere and form the granules. The granules would then be dried and stored.
On its own, Couscous is fairly bland -- just as rice and potatoes would be if you served them on their own. But like rice and potatoes, it is used as a side for some dishes, and as the basis of others.
Purists will maintain that, just as they do in North Africa, Couscous should always be steamed and never boiled. There's lot of other things they do in North Africa that would get you nicked for doing here. Besides that, I don't suppose you'll see these purists actually making their Couscous by hand, the way some still do in the Middle East and Mediterranean Africa.
In the Middle East, Couscous is usually served with savoury stews called Tagines -- serving the same function, in a way, as our mashed potatoes do in soaking up gravy.
In 2006, a survey conducted for the French magazine VSD found that couscous was the favourite dish in France. Paella, pizza and spaghetti bolognaise also ranked highly. 
There are two ways to prepare Couscous: steaming it, or pouring boiling water on it and letting it sit to re-hydrate. The standing method is handy if you don't happen to have a stove with 8 burners on it and the burners you do have are already claimed by other pots. You can use boiling water or stock. Pour enough on to cover the Couscous and have 1/4 inch (5 mm) of liquid on it beyond that. Cover it with a plate or plastic wrap, and let sit for about 10 minutes, then fluff with a fork. It should be soft, and if it isn't, add a little more boiling water, mix it in, and then re-cover for a few more minutes.
The Couscous shouldn't be mushy, or very wet; the granules should be tender to the bite and should separate from each other when forked.
Once it is cooked, you can then use it as you need to in your recipe. To use it as a side, add some olive oil or butter and anything else that strikes your fancy to tart it up.
You will read in a few places that an average serving should be 1 oz (30g), but these people must be feeding Pixies. Allow instead 1/2 cup / 3 oz / 100g of uncooked couscous per person.
You can make a fancy stuffing for poultry and meat out of cooked Couscous.
And, as a helpful hint for the Purists, in Morocco, Couscous is actually cooked 3 times. It's rinsed in cold water, steamed for 5 minutes, then the process is repeated twice more. We'll expect them to get right on it.
1 cup uncooked couscous = 7 oz uncooked = 200g uncooked = 2 1/2 - 3 cups cooked
Will keep at room temperature in a tightly-sealed bag or container.
Couscous has been made in North Africa for over 1,400 years. About the 9th century AD, the Arabs introduced it to parts of Sicily and in those parts it remains part of the daily cooking.
Couscous has been manufactured in the United States since 1993.
"Extract of a letter from Gibraltar, May 16 . "A few days ago arrived his majesty's cutter, captain Ferritur, from Tetuan [Ed: Tetouan, Morocco], who says, that when he was there, it being the Moorish festival, the governor, and all the heads of the town, dined with Joseph Popham, Esq, his majesty's consul-general; there was in the middle of the table a large dish of Cusscussa the general food of the country, the rest of the victuals was in the English manner; their drink was butter-milk and lemonade; there was no wine at the table for fear of giving offence, it being contrary to their law to drink wine." -- London Annual Register. London, England. 1 January 1761. Page 130.
Literature & Lore
It's not entirely certain where the word "Couscous" came from; one of the more plausible explanations attributes it to an Arabic dialect word -- kaskasah -- meaning to grind or pound.
 Randall, Colin. French abandon traditional cuisine in favour of couscous. London: Daily Telegraph. 31 March 2006.
CouscousCouscous; Israeli Couscous; Mograbia
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Cuscus (Italian); Cuscus (Portuguese)