没有搞混啊, 你看, 人家定义的非常清楚。
In Parliament, the majority party selects its prime minister, who is the official head of government and also serves with his cabinet members of the legislature. What this means is that members of Parliament aren't able to make independent decisions from what the majority party and the head of government want because they are all part of one lawmaking body. Put simpler, because the head of state (the executive branch) and the Parliament (the legislative branch) are so intermixed, they must always vote to support each other or else the government will not succeed.
In Congress, the members have more freedom. The president, who is the official head of government, is elected popularly by the people, and he independently chooses members of his cabinet from people that are not members of Congress. The head of government does not participate in the legislature. The members of Congress, although some may be members of the same political party as the president, do not have to answer to him directly and can make independent decisions.
Put simpler, because the head of state and Congress are separated, the government is protected by a system of checks and balances because Congress and the president do not always agree and have the power to disagree with, or, in some cases, vote to change, decisions that the other makes. This makes our government stronger.