A kayak is a small, narrow human-powered boat traditionally built with an enclosed deck and one or more cockpits; the paddler sits low with legs forward and propels it with a long double-bladed paddle. That enclosed deck is what most clearly separates a classic kayak from an open-deck canoe, though modern sit-on-top kayaks leave the paddler on an open deck while still using kayak-style paddling.
A canoe is usually open on top, often wider, with the paddler sitting or kneeling higher and using a single-bladed paddle (switching sides or using corrective strokes to track straight). In short: kayaks are generally narrower and double-bladed; canoes are more open and single-bladed.
Source: Wikipedia — Kayak.