The bureaucracy is a large group of people whose role is to ensure that laws are executed; it creates rules to enable this; and in adjudication. The President’s role is to take care that laws are faithfully executed. The bills introduced by the Congress and made into laws by the President are implemented by the bureaucracy. For this bureaucrats write rules specific to the proper implementation of each law. In case there are any disputes while the rules are being created for the proper execution of the laws the bureaucracy has to adjudicate in the disputes.
Most of the bureaucracy worked under the Spoils System during the 19th and the 20th century. Here what mattered was not how well people worked but who they knew. Under the Merit system people are hired and promoted based on their qualifications and the performance of their job instead of their political connections.