Actually, the flu is a viral disease called influenza. It may have symptoms in common with a cold, but it's more serious, since it can have potentially fatal complications. The flu kills 20,000 Americans each year. For the most part, the distinction between colds and flu is not especially significant. In healthy adults younger than 65, both diseases are usually self-limiting, meaning they'll go away on their own.
Antibiotics are useless treatments for both.
Depending on which strain of the virus you catch, the flu generally packs a meaner punch. Its symptoms tend to be more severe and it may last one to two weeks, as opposed to a few days to a week for a cold. The real reason to learn the difference is the possible after-effects. A cold may sometimes lead to a secondary bacterial infection, treatable with antibiotics. But for older people and those with weak immune systems, influenza carries the potential for deadly complications.