Related to the onion (as opposed to being a younger version of it), shallots grow in clusters at the leaf base. Most varieties are smaller than onions, have finer layers and contain less water.
The flavour of a shallot is much milder and sweeter than that of an onion, so if a recipe specifies shallots, substituting onions won't give the same results. Their lower water content means they need to be cooked more gently than onions.
Weight is typically 250g