Cannelloni (also called manicotti in North America) are tubular pasta shapes, or flat pieces of fresh pasta rolled into tubes. They may have a smooth surface, especially if they're homemade, or they may have ridges on the outer surface if manufactured. I think the ridges help the sauce to adhere to the noodles. In any case, cannelloni noodles are are stuffed, topped with a sauce, and then baked, for a tasty main dish. Cannelloni are usually a bit larger than manicotti. If you want to make cannelloni by hand, see the recipe at pasta and the directions at how to roll out fresh pasta, and thank the lords above for the expertise of sneff. Or, do like I do and buy it dried, in packages.
In keeping with the new trend towards ultra-"convenience" foods, you can now buy packaged cannelloni (and manicotti and giant shells and lasagna noodles) which do not need to be cooked in boiling water first. Just stuff, surround with sauce, and bake! says the package. But note that you need a lot of quite thin sauce to properly soften the noodles. Having had some tough, undercooked noodles, I prefer to buy the type that needs cooking in boiling water first. Just remember that the cannelloni will be baked after cooking in boiling water, so cook it until barely tender.
The stuffing for cannelloni can be anything at all, really, but a spinach cheese filling is pretty standard in Italian restaurants in North America. To make this, first steam a bunch of spinach and let cool; then squeeze out the excess moisture and chop. Meanwhile, saute an onion till soft, add a clove of garlic and saute till fragrant, then let cool. Combine spinach, onion and garlic, and add ricotta and parmesan cheeses, or tofu if you're vegan. Stuff the cannelloni, place in a single layer in a baking dish, top with a hearty tomato sauce, and sprinkle with more parmesan or mozzarella cheese, if you like. Cover the dish with foil, and bake 350°F (180°C) for about 40 minutes. Let stand for 5 minutes, and serve hot.