What terms describe the government of the untited states?

The US system is defined as a 'Presidential system' as opposed to the 'Parliamentary system'. In a Presidential system like that of the USA, France and several South American countries, the President has large executive and often military powers, and he can influence the working of his Parliament / Congress by vetoing laws, and of his Judiciary by appointing the Supreme Court judges. It is a system that can work well in a democratic system like the US, but it can also easily lead (as it has done in most South American systems at some point) to Presidents assuming dictatorial powers.

In a Parliamentary system like the UK, Germany and most other European countries the Parliament is the highest power in the land; it can send away any Secretaries of State it doesn't like or trust, including the Prime Minister, and the head of State (President or King) in a Parliamentary system has mostly ceremonial tasks.