At the federal level, two agencies of the Department of Labor are the most important in terms of workplace regulations. The Employment Standards Administration enforces the Fair Labor Standards Act (FLSA), which mandates labor standards such as breaks, overtime, and minimum wages.
Another agency of the Department of Labor, the Occupational Safety and Health Administration (OSHA) regulates workplace safety. There are also other agencies of the Department of Labor that are concerned with more limited workplace issues or particular industries, such as the Employee Benefits Security Administration and the Mine Safety and Health Administration.
Most states have their own Labor departments and often agencies very similar to the Employment Standards Administration and OSHA (the states equivalents are often called "baby OSHAs") that are charged with enforcing the equivalent state regulations. In many cases, state regulations may exceed those of the federal government in terms of providing a higher minimum wage or more extensive workplace safety rules.