How much detail do you want?
The Executive Branch, in the US, is the President and all the powers/limitations granted to him by the Constitution. The bulk of the Branch itself in made up of the Bureaucracy.
The Bureaucracy is the system that allows the Executive branch, in fact all branches, of the Government to run. It is generally made up of mid to high grade federal employees who are not political appointees but civil servants. They are the one who actually make the wheels turn in the background and the one who stay where they are despite the political wind and which way it's blowing. They, in essence, implement the basic wishes and will of those in high positions to make things happen. Everything from booking flights to making copies to making coffee to calling companies and contracting services, the Bureaucracy makes it happen so the whole system runs.
Any large scale organization, including companies, have a Bureaucracy that runs the place and makes the day to day operations happen.
Probably not the answer you wanted though...
