Sardine is a generic term applied to any one of a number of small, soft-boned salt water fish like herring, sprat, or pilchard. The name apparently derives from the place where young pilchard became one of the first fish packed in oil: Sardinia.
Sardines are usually silvery and iridescent in colour; they swim in huge schools near the surface of the water. They are fairly fatty, so when fresh are usually grilled, broiled, or fried. More commonly, in North America at least, they are salted, smoked or canned, usually packed in oil or tomato sauce. They're usually packed without heads or tails but with bones intact, in which form they are a good source of calcium for the lactose intolerant. They are also one of the few natural sources of vitamin D, and the combination of vitamin D and calcium helps lessen your risk of osteoporosis. Sometimes sardines are skinned and boned and sold as fillets; not so rich in calcium, but still an excellent source of Omega-3 fatty acids.