A motor that you attach to a boat and can remove, as opposed to an inboard motor that's built in. Comes in two flavours: gas powered and electric. Johnson and Evinrude are the two big brands.

Gasoline comes in big red metal tanks that hold 5 gallons. You squeeze a bulb that's at the gas tank end of the fuel line in order to get the gas moving. Electricity comes via a marine battery, which is basically a well-sealed 12-volt car battery. The advantage of electric motors is that they're almost completely silent, which is good if you want the pristine woods unsullied with noise, or if you want to sneak up on fish in order to catch them. You won't go as fast as with a gasoline motor, though.

The output is measured in horsepower, which determines the price. A 4-hp is perfect for putting along in your 12-foot skiff. For a 20-foot bass boat, you want a 35-hp. And if you want to pull water skiers, you're looking at 60- or 70-hp.