The term Department usually denotes the parent organization. The U.S. Department of Justice, the New York Police Department, the U.S. Department of Homeland Security, the Los Angeles Sheriff’s Department are all examples of this. The term Office may be used as a synonym for Department, especially when the head of that organizational group holds a publicly elected Office. A Sheriff is an elected official, and it is somewhat common to refer to a Sheriff’s Department as a Sheriff’s Office.
At the Federal level, the terms Bureau, Agency, Administration and Service usually indicate an individual agency within a Department. The Transportation Security Administration (TSA), the U.S. Secret Service, and the Federal Emergency Management Agency (FEMA) are all individual agencies within the U.S. Department of Homeland Security. The Drug Enforcement Administration (DEA), the Bureau of Prisons, and the U.S. Marshals Service are agencies within the U.S. Department of Justice.
[SPELLING ALERT: There is only one letter “L” in United States Marshals Service.]
Artistic license grants you the ability to dream up any name you desire for these organizational subdivisions. Being aware that the terms department, division, bureau, unit, and team are not necessarily synonyms may be a subtle way to add realism to your work. Understanding these delineations between groups may help you add conflict or explain why certain characters are (or are not) privy to pertinent information that will drive your plot forward.
What Departments and Units are your characters working in? Let me know in the comments below.