In political systems based on the principle of separation of powers, authority is distributed among several branches (executive, legislative, judicial) — an attempt to prevent the concentration of power in the hands of a small group of people. In such a system, the executive does not pass laws (the role of the legislature) or interpret them (the role of the judiciary). Instead, the executive enforces the law as written by the legislature and interpreted by the judiciary. The executive can be the source of certain types of law, such as a decree or executive order. Executive bureaucracies are commonly the source of regulations.
In the Westminster political system, the principle of separation of powers in not as entrenched. Members of the executive, called ministers, are also members of the legislature, and hence play an important part in both the writing and enforcing of law.
An executive officer (often abbreviated XO) is generally a person responsible for running an organization, although the exact nature of the role varies depending on the organization.
While there is no clear line between executive or principal and inferior officers, principal officers are high-level officials in the executive branch of U.S. government such as department heads of independent agencies. In Humphrey's Executor v. United States, 295 U.S. 602 (1935), the Court distinguished between executive officers and quasi-legislative or quasi-judicial officers by stating that the former serve at the pleasure of the president and may be removed at his discretion. The latter may be removed only with procedures consistent with statutory conditions enacted by Congress. The decision by the Court was that the Federal Trade Commission was a quasi-legislative body because of other powers it had, and therefore the president could not fire an FTC member for political reasons. Congress can’t retain removal power over officials with executive function (Bowsher v. Synar). However, statutes can restrict removal if not purely executive (Humphrey’s executor), but can't restrict removal of purely executive officer (Myers v. United States, 272 U.S. 52 (1926)). The standard is whether restriction "impedes the president’s ability to perform his constitutional duty" (Morrison v. Olson, 487 U.S. 654 (1988)).