Just as there are many types of cyclist, so too are there different types of cycling jerseys and tops.
The short sleeved jersey can be worn almost as you would a T-shirt, by its self, or beneath a jacket. But where a cotton T-shirt can chafe and be uncomfortable, a short sleeve cycling jersey will use a technical fabric designed to wick away sweat from your skin, or even from a base layer.
Long sleeved jerseys are often designed for warmth and are used as a mid-layer between a base layer and a jacket. These long sleeved layers will come in different weights and provide different levels of insulation, and although they use pretty good wicking fabrics, they perform best in when paired with a base layer.
Some jerseys may even have windproof properties. Weather an off-road cyclist in a more upright position, wearing something with a looser fit, or a road racer, in a far more aggressive position, wearing something much more fitted.
The cycling jersey has the riders cycling position in mind.