Couscous, if you've never tried it, is a north African dish consisting of semolina grain (like a fine-grained pasta in texture)
served either vegetarian or with meat, lots of vegetables (carrots, turnips, potatoes) soaked in broth.
There's also a red spicy paste served with it that you can stir in to your dish to make it as spicy as you like.

Couscous Nature is straight up vegetarian couscous
(although I'm sure the broth is a meat broth as the Parisians don't usually "get" what straight-up vegetarianism is - no meat period),
Merguez is a lamb sausage (delicious, too),
Poulet is chicken, Mouton is sheep (mutton),
Gigot is leg of lamb,
and the Royal is a mixture of meats (as you can see from the sign).

Here's a photo I took from my last trip to Paris of a Couscous Merguez I tried on the Place de la Contrescarpe.
http://www.pbase.com/image/57547828