The hull is a kayak's underside. Its shape and width determine stability. Sea kayaks have more of a V-shaped hull, while white water kayaks have more of a rounded hull.
Front and Back
The front of a kayak is called the fore end and the back is called the after end, or aft. The front of the kayak that is in the water is called the bow, and the back of the kayak that is in the water is the stern.
Where You Sit
The area in which you sit in a kayak is called the cockpit, and the rim around this area on which you affix your spray skirt is the cockpit coaming. The rim around the boat where the deck ends and the hull begins is the gunwale.
Skeg and Rudder
Some kayaks have a skeg, like a fin, many of which are now designed to be retractable on the aft end of the hull. This helps a kayak run straight in the wind. Rudders are used similarly to help maintain course, and can be shifted, left or right, by foot-operated pedals in the cockpit.
Atop the Deck
Atop the deck of the kayak should be the fore deck and rear deck elastics, which are used to hold gear, tow lines, tow line anchor point (behind the cockpit), and an end toggle at each tip of the boat for carrying. Some kayaks also have a foredeck compass.