Why is the united states of america described as "democratic, federal, republic?

The main reason why the United States is often described with these terms is that they are fairly interchangable. Democracies are often federal, which is the same as a republic. 

Ghana, first of the great medieval trading empires of western africa (fl. 7th–13th century). it was situated between the sahara and the headwaters of the sénégal and niger rivers, in an area that now comprises southeastern mauritania and part of mali.