When buying an extension cord, decide what it will be used for before choosing which cord. Wire size is very important. A 14 gauge cord is rated for a maximum of 15 amps, and 18 gauge cords are rated for a maximum of 10 amps. Always buy a larger gauge cord than you think you will need.
Another important factor is the length of the cord. Due to a phenomenon called Voltage Drop, you get less current out of a cord than you put in. The longer the cord, the greater the voltage drop. Buy the shortest cord you can get by with.