Canoes are available in several kinds of materials and with different construction methods:
Royalex is a very tough and durable but still a reasonably light weight plastic laminate. A 17 foot Royalex canoe will weigh about 64 pounds compared to 75-85 pounds for other plastic or aluminum.
Composite materials include Tufweave, a fiberglass and polyester hybrid that outperforms ordinary fiberglass types S or E. Kevlar is a an extremely tough material that is also very light weight. Graphite is even lighter than kevlar but not as abrasion resistant.
Flexcore construction uses several layers of either Tufweave or Kevlar with a specially designed hull reinforcement in the bottom of the canoe. This reinforcement is designed to be quite stiff to retain the design shape but able to flex rather than break on obstacles. This is a very tough construction method but not as tough as Royalex. It's advantages are lighter weight and better performance. It is easily repaired if you manage to damage it.
Ultra light construction uses fewer layers of specially reinforced Tufweave, kevlar and or graphite to build the lightest canoe that is still practical for everyday use in reasonably demanding conditions. It usually does not have a color applied to the hull unless you ask for it. The gel coat (color) adds about 5 pounds to the canoe. This is the stiffest construction method, resulting in the best performance. It is not a good choice for a steady diet of hitting rocks and gravel unless light weight is a very high priority for you. It is especially good for lake and large river trips where carrying the canoe is over portage routes is necessary. Some smaller people and those with less upper body strength appreciate the light weight in all circumstances. It is reasonably easy to repair in most cases.