I like Retro Grouch's description as well. The less you tell the customer, the better off you will be. Keep it as simple as possible but still cover the basics. Tell them too much or get too technical, their eyes will glaze over or they will feel intimidated and you may lose the sale.

Our biggest customer right now hasn't been on a bike in forever and is most likely shopping for a comfort, hybrid, or sport road bike of some type. Usually with 21 or 24 gears. I tell them that if you can ride a 3 or 5 speed you will have no problem with this. I give them a quick "High, medium, and low" description of the front chainrings first. Select that first depending on your terrain and then work the gears in the back afterward. I show them how the shifters work, which way to go for easy or harder gears, must be pedaling while changing gears, and that's about it. A lot of times tossing the bike in a stand and showing the customer the mechanics of what's going on can work wonders for simplifying things.
Reminding the customer about cross-chaining and easing up a little on the pedals while shifting is added in when sending them out for a test ride.
I stay away from the car analogy as that may conjure up memories of double clutching and the three speed column shift of the customer's first car that they hated. I don't want them thinking "if that car only had three gears imagine how difficult 24 will be..."