The difference between a short and long action is usually attributed to the cartridge the rifle is chambered for. Short action rifles are chambered in calibers such as .223, .243, 22-250, and .308, which are fairly short cartridges compared to say .300WM and 30-06, which are chambered in long actions.
If you're going to buy a rifle, you usually don't think about whether it's a short or long action, because most of the time you're looking for a particular caliber rifle. The only time you would consider the action length is if you plan on rebarrelling/rechambering for another caliber in the future.