The president is the head of the executive branch of the federal government. His job is to sign bills passed by Congress into laws, enforce federal laws, and command the military, amongst other things. As the number of laws and the government in general grows, that becomes more and more work for one person to handle. So, over the years, presidents have established new departments in the executive branch to help run the country. The heads of these departments, along with the vice president, make up what is called the Cabinet.
The Cabinet of the United States originally consisted of the vice president, the attorney general, the secretary of state, the secretary of treasury, and the secretary of war – which was later renamed secretary of defense. Over the years, many new departments – and therefore Cabinet positions – have been added. The full Cabinet now includes the following departments: Interior, Agriculture, Commerce, Labor, Health and Human Services, Housing and Urban Development, Transportation, Energy, Education, Veterans Affairs, and Homeland Security.