Onions and Scallions are two examples of vegetables. Scallions are also onions, and they are most times called with different names. Scallions are known as spring onions, stick onions, green onions, etc. Scallion belongs to genus Allium, but its specie varies, and that's why we can have the different names listed above. On the other hand, Onion also belongs to genus Allium, and its specie is Allium Cepa, although there are other species of onions; Japanese onions, Egyptian onions, etc.
Onions are also known as garden onions or bulb onions. Another difference between scallions and onions can be noticed when you consider their appearance. Scallions do not have bulbs like onions, they are harvested once they reach maturity, and at this stage, they have long green leaves. That's why in some parts of the world, they are grown mainly as salad onions. Onions, on the other hand, are bigger than scallions, this is because they are harvested only when they have big matured bulbs.