The American government does a lot of things with regard to foreign policy -- everything from war and espionage to foreign aid.
The government is the main actor in foreign policy. It is the government (mostly the executive branch) that decides what America's foreign policy will be. It decides, for example, whether it is our policy to try to overthrow Iran's government or not. Then, it takes steps to implement these policies. For example, if we decide we want Iran's government out, we might try to persuade the UN to place sanctions on them for their nuclear program. Or we might try to support groups within Iran that are trying to get more democracy in that country. Both of these are things that the government does in relation to foreign policy.