It comes in paste or powdered form, either in a tin or a tube. In France, the tube form is more common, and is usually served as a condiment for couscous.
Harissa is made by blending smoked red chiles with cumin, coriander, caraway, and olive oil. Some brands contain tomatoes or rose petals. Harissa is used in both appetizers and main courses. It's used as a condiment with olives, tuna, and tomatoes. It's also used as a sauce in many North African dishes and stews, and as a marinade or rub for meats. It's also used as a sauce for spaghetti or pizzas.
Harissa can be found at Middle Eastern grocers, specialty food stores, or online. A good online deal is this Crate & Barrel listing for $5.95.