The United States Constitution has a formal Separation of Powers that divides government into three branches, giving each branch checks and balances on the others to prevent one from becoming too powerful. For example, the legislative branch can impeach the executive branch in the event of wrongdoing.
The legislative branch is also able to override presidential vetoes, approve of various appointments and declare war. The executive branch can check the legislative branch with vetoes and calling emergency sessions of Congress. The executive branch appoints members of the judicial branch, which can also be considered a check. The judicial branch is able to perform judicial reviews on either of the other branches.