Look for firm shallots, with no soft spots, damp or mouldy patches. Brown (also known as English or Dutch) shallots are the most commonly available. Small, with a light brown skin, they have a mild flavour, and sometimes have more than one bulb inside.
Banana shallots are the largest variety and are named for their size. They have a smooth, tan-coloured skin and are slightly milder than other types. As they're larger, they're swifter to prepare than the same volume of smaller shallots.