If you are new to skatebaording, you should learn about skateboards: shortboards vs. longboards. It would be very embarrassing to confuse a longboard with a shortboard. Additionally, different board lengths serve different purposes. You don't want to show up at a skate park with your longboard; conversely, you don't want to be strolling down the street on a board that is designed for the half pipe.
The obvious difference between the longboard and the shortboard is their respective lengths. A longboard is not just a longer skateboarding deck. The longboard refers to a surfboard-like deck. The shortboard refers to everything else and can be used interchangeably with "skateboard." Even though skateboards vary slightly in length, they won't ever be called "longboards."
Another area where shortboards and longboards differ is their use. Longboards are especially popular for downhill racing. The larger wheels and trucks on the longboard help it to gain momentum, where the wheels on a shortboard are designed to keep it from going too fast. The longboard generally has more stability than the shortboard and that is why it is used as a vehicle on many college campuses. Longboards are too long for doing tricks, imagine trying to execute a kick-flip a surfboard-like object with wheels. Short boards are more lightweight and are built to be responsive.
There are also different stereotypes for people who ride longboards versus shortboards. People who ride longboards tend to be more of the hippie, surfer types. People who ride shortboards tend to be associated with the punk scene. They tend to be younger and more rebellious. More men ride shortboards and it is less common to see women riding shortboards. There isn't a different between the number of men and women who longboard.