Forms bunches of bulbs. In spring you set shallot sets and they will then split into several bulbs as garlic or potato onions do. Normally you get 4-8 shallots per set. There are varieties with round shallots, and those with long shallots. If you use big sets you usually get many small shallots, whereas if you use small sets you usually get fewer but bigger shallots. Many varieties of shallots flower very seldom. If you want to save your own sets, it's easy to just save some of the smaller bulbs in a paper bag that you hang somewhere that's not heated.
It's hard to draw the line between shallots and potato onions, but they often have a different taste and potato onions are usually round.