Related to the onion, shallots are generally smaller, milder and slightly sweeter. They need less vigorous cooking as they have a lower water content. Kept in a cool, dark place - they should last you several weeks.
Shallots actually have a better nutritional profile than onions and garlic with a higher concentration of flavonols and polyphenolic compounds which are believed to be responsible for their antioxidant properties. Studies have connected shallots to a reduction in lung and oral cancers as well as stomach, colorectal and breast cancer.