Materials and weight:
"Sheet": These are materials generally made from extruded material - vinyl, polypropylene or other similar material. Generally made of several layers and heat formed into shape. In the past tended to be heavier but more resilient than composite. Royalex was the material in this class for years, but the single manufacture stopped producing it. There are now a number of companies competing with new materials in the canoe market.
"Composites": Combination of woven cloth and resins of various types, including fiberglass, Kevler/Armird, basalt, Integra and others. Canoe builders use one or more types of cloth in a single boat to provide a range of choices in terms of cost, weight, stiffness and durability. In general tend to be lighter and stiffer than "sheet" boats.
"Other": Grumman is still making aluminum canoes, and there are still companies making traditional wood and canvas boats. You can find kits and instructions for wood stitch-and-glue, strip built and skin on frame canoes, as well as other companies selling folding and inflatables. There are lots of choices out there.