A social security card is an official document issued by the U.S. government that contains a unique nine-digit number assigned to individuals. This number is used primarily for tracking earnings and benefits within the Social Security system, which provides financial support to retirees, disabled individuals, and survivors of deceased workers. The social security number (SSN) on the card is also commonly used for various purposes, such as opening bank accounts, applying for loans, and filing taxes. It is important to keep the card safe, as the SSN can be a target for identity theft if it falls into the wrong hands.