Read our review of the Primal Chameleon Women’s Evo 2.0 Cycling Jersey,Compared to a t-shirt, a cycling jersey usually has a longer back, shorter front a higher neck and sleeves shaped to fit when you're reaching for the handlebars.
There's a lot of variation in how closely a cycling jersey fits. Some are very close and tight-fitting,cheap cycling jerseys especially if they're intended for racing. You don't want excess fabric flapping in the breeze if you're trying to go as fast as possible. This is usually called something like a 'race cut', so avoid jerseys with this kind of description if you want something with a bit more give... similarly, you wouldn't buy jeans advertised as 'skinny' if you like them baggy!
Other cycling jerseys are looser and a more leisurely fit if you're not a racer. Look out for 'sport', 'city' or 'casual' designations.