Shallots are closely related to onions but tend to have a milder and sweeter flavor. They are also related to garlic and are similarly shaped.
Good shallots are firm and dense. Check for bruises and avoid overly dry shallots. Larger shallots will be stronger while smaller shallots tend to be milder.
Shallots can be used as a substitute for onions when a milder flavor is desired. Depending on how you like them, they can be grilled, fried, and even pickled.