US citizens, what's difference the between these governmental organizations

what is the defee between (position in government structure/what does it do):

  • A department (for ex.:The State Department)& this department (for ex.: the NYPD).
  • A Bureau (for ex.: the FBI).
  • An Agency (for ex.: the CIA).
  • An administration (for ex.: The DEA).
  • A Center(for ex.: the CDC).
  • A Service (for ex.: the DSS).
  • A Unit (for ex.: Criminal Investigation Unit).
  • A Commission (for ex.: Nuclear Regulatory Commission).
    thnx in advance

Our government is like an octopus and has many tentacles and they each like to use a different descriptive verb.
Regardless of the name they use, they are still under the control of the main governmental bodies.

I hate to admit I really didn’t know the differences. There are definitions of some of the terms here

http://www.access.gpo.gov/su_docs/fdlp/pubs/classman/cl_gl.html

while some, like “Center” might not have a firm definition.

I think in terms of local government for “Unit” but it may be broader than this.