Arborio rice is a variety of short-grained rice used primarily in the classical preparation of risotto. Originally cultivated in Italy, today it's grown in California and Texas, too.
Short, fat and slightly oval-shaped (pictured in the lower-left of the photo here), arborio rice has a pearly white exterior. There are various size designations, of which superfino, the largest grain size, is the one most commonly used in the United States.
Because it undergoes less milling than ordinary long-grained rice, it retains more of its natural starch content. Cooking releases this starch, giving risotto its creamy consistency. A pound of arborio rice can absorb up to 6 cups of liquid without becoming mushy.
This process of releasing is starch is key to risotto's creaminess, and it's a process that only happens if it's cooked slowly, with the liquid added a little bit at a time. If you were to prepare arborio rice via the traditional method for cooking white rice, where all the water and all the rice are combined in a pot and then simmered until the water is absorbed, you'd get cooked rice, but not the same creaminess.
And releasing this starch takes time. This is one of the problems with risotto when you order it in a restaurant. Preparing risotto requires a minimum of 20 minutes of constant stirring and gradually adding the liquid.
Thus, not only is it labor intensive, but 20 minutes is far longer than most restaurant patrons are willing to wait for their food.
And crucially, once you've made the risotto, you can't hold it for any length of time as the starch will immediately start to congeal, causing it to turn stiff and gluey.
The solution for most restaurants is to cook the rice part way (known as parcooking), and then holding it, and when an order comes in, the cook can go to work finishing the cooking in the usual way, adding the liquid a little at a time while stirring.
In this way, the order can be sent out within 5-10 minutes. And naturally, it won't be as good, which is why so many people have mixed experiences when ordering risotto at a restaurant.
Like pasta, arborio rice is prepared al dente, which means that it should be slightly firm to the bite — which is a little bit less done than you would cook ordinary white rice.
Also, here's a step-by-step tutorial on how to make risotto.