A cantenna is the name given to a directional antenna that is constructed out of a metallic can. Most commonly used for wireless ethernet purposes, although I'm sure it could be used for anything that uses RF.
The most publicized can used in a cantenna is a Pringles can, although simpler ones can be made out of soup cans or any kind of metal can. The term cantenna is not specific to a particular design of antenna. Some are yagi-style antennas--the Pringles can design with is the most widely known is one of these. The simpler ones are waveguide antennas, which are easier to make and can actually work just as well in practice.
I have a cantenna which I constructed out of a used can of beans, and it gives me roughly a 18dBi boost in signal, according to the Orinoco link test utility.