Most canoes you see are for recreational use with two paddlers. Solo canoes are shorter and lighter than those designed for tandem paddling. They are usually about 13-14 feet long, but longer lengths are common in canoes where speed is important (i.e., cruising or racing).
Canoes are made from a variety of materials. Aluminium canoes are quite popular because of their low cost and ability to take heavy wear, but like your grandfathers' old '67 Olds, are not very efficient. Fiberglas is also popular, as are royalex (a foam/plastic composite) and some kevlar (light, but very expensive) boats. Although aluminium canoes are less expensive, they are not often a bargain. You cannot produce desired hull shapes with aluminium, and they are heavy. Mowhawk Canoe makes a decent fiberglas canoe for about the same price as an aluminium boat. In defense of aluminium, it is almost indestructable, and is a little lighter than ABS plastic.
Alumnium has a few other bad points as well: it's cold in the early morning, which matters to wilderness trippers who rise at dawn and glide out onto a misty lake before summer's heat notices them; and it's noisy in even tiny little wavelets, which makes all that gliding over a misty lake so much less romantic. Also a dull gray powdery guck rubs off on you. One advantage: nothing you can do to it will make it uglier. :-)
Besides, it makes you look like you're in a rented canoe.
So, keep the cold and the powdery guck in mind if someone tries to sell you a fibreglass/plastic canoe with aluminum gunwales. Stripper (fibreglass cloth over thin strips of cedar) canoes are beautiful, weigh less than aluminum or fibreglass, and are strong too. But you pretty much have to make one yourself.
Decked canoes are a different animal altogether from open canoes previously discussed. A decked boat characterized by two features: the paddler is in a kneeling position, and uses a single-bladed paddle. Single-seaters are known as C-1's, doubles as C-2's.
Decked canoes and kayaks are capable of negotiating just about any runnable water when paddled skillfully. There seems to be a general consensus that the learning curve for C-boats is flatter than that of kayaks. The problem is that because the paddler uses a single bladed paddle, there is the lack of a good strong brace on one side, and that takes a while to develop. Advantages are a very strong stroke, bringing in more big back and trunk muscles than kayaks can, because of the position of the paddler. They roll easily, are are extremely agile, and good paddlers beat kayaks sometimes. Jon Lugbill routinely beats kayakers in identical situations. Because of their shape, they are often mistaken by the ignorant bystanders as "kayaks" ["Hey! How come you're using that canoe paddle in that kayak?"...]
I like the way it makes my toes cramp up after a long day.... :-)