Sketches courtesy of Tel Monks. Dave firstname.lastname@example.org has converted them into a nice animation (68K). I suggest you take a look at the animation, then come back and read this page. [Please note that the sketches and animation are incorrect in one respect. In a real lock, the gates open the other way, toward the high water side.]
Referring to the picture and sketches, visualize two bodies of water, one whose surface is, say, 8 feet above the other. Visualize a "tank" (the lock) between the two bodies of water. The tank extends a few feet above the higher water surface. Now suppose we are cruising along in our boat on the upper body of water. Suppose also that the water level in the tank is at the same level as we are. Then there is no problem in opening the door (gate) on our side of the tank, and driving ourselves in.
We close the door behind us. We are now floating in the closed tank. Now what? Well, using an interesting contrivance (a "paddle"), one of our crew starts to let water flow out of the tank into the lower body of water, sort of like emptying a bathtub. As the water level in the tank goes down, our boat naturally descends with it.
When the tank is finally drained to the same level as the lower body of water, we can open the door on that side (another "gate"), and steer our boat out.
For going up, we steer our boat into the drained lock, close the gate, allow water from the upper body of water to flow in. When the levels match, we open the upper gate and drive out.
Now that you've gone through the explanation, you can try to do it yourself using an interactive lock animation, by James Pascoe. Just replace his word "door" with "gate", and his word "sluice" with "paddle". As long as you have not disabled Java on your browser, you should be able to use the animation. (Note that this animation has the same problem as the previous animation and sketches - the gates open in the wrong direction-- but the basic principles are still intact.)
Another relative rarity on the canals are "side ponds" associated with locks. The side ponds allow a full lock cycle (from full to empty to full again) to be done WITHOUT using a whole lockfull of water. This seven minute video on Using Lock Side Ponds shows how this works.
[Photo: believe to be the second lock from the bottom of the Atherstone flight on the Coventry Canal.]
Read about how Joan and Pete Payzant first learned how to operate a lock.
Nigel Bromley has a detailed page on Canal Locks - collected thoughts on their theory
Very thorough coverage of locks, in all their variations, may be found on the Wikipedia page Lock (water transport).
Lockkeepers in France only require a very small tip for their services. They are well known to offer items for sale: home-made goat's cheese, eggs, fruit, and vegetables.
For an example of lock repair, see Photographs of Stenson Lock's bottom gates being replaced