Shallot are about halfway between garlic and onions. The flavour is more like garlic than an onion is. They don't have the same bite most cooking onions do and are not as strong as garlic is.

They grow more like a garlic because onions grow a single bulb but shallot grow a clump of offset bulbs. The offset bulbs of shallot are not as tightly bound into a head as garlic cloves are and usually are larger than garlic cloves. There are usually fewer of shallot bulbs in the clump than there are cloves in most garlic heads.

If you do not have shallot you could substitute green or clumping onions but the flavour will be less garlic than shallot gives.
If you mixed some sweet onions and a very small amount of garlic you might come close to the effect of shallot.

Depending on what you are cooking it might not matter much if you are using shallot or onion. Shallot can be expensive but it is not very hard to grow if you have a garden.

Sono entrambe verdure a bulbo della famiglia Allium insieme all'aglio e ai porri Gli scalogni sono più piccoli e penso che il sapore della cipolla sia più acuto e concentrato.