An FBI file is a dossier of information kept by the US Federal Bureau of Investigation (FBI). Such files are used to keep track of persons and organizations of interest for the FBI, and many people have such files, although the contents may not be terribly interesting. In the early 21st century, thanks to legislation such as the Patriot Act, many intelligence agencies including the FBI established systems for sharing information between each other, potentially making such record keeping more efficient.
There are a number of reasons for the FBI to open a file on someone. Both American and foreign citizens have files at the FBI, and the intelligence agency keeps extensive files on many prominent people. As a general rule, any sort of activity which could potentially pose a threat to national security is recorded in an FBI file. For example, someone who regularly attends protests may have an FBI file, as the intelligence agency may identify this person as a potential radical. Obviously, actual radicals also tend to have extensive FBI files which record their activities, known associates, and so forth.
The information in an FBI file can be quite varied and very extensive. Agents typically add information as they acquire it, making notes about the movements of a subject and the people he or she interacts with. FBI files can also include transcripts of wiretaps, subpoenaed bank records, and other information which has been gathered on an individual.
Along with other agencies, the FBI has a large database of declassified files on historically interesting people. The intelligence agency maintains an electronic reading room for people who want to read FBI files on people like Albert Einstein, Groucho Marx, and the Zodiac Killer, along with organizations such as Greenpeace, among many others. Some of these documents may have redacted portions, but they are still quite interesting to read, and they can provide some interesting historical context.
It is also possible to obtain a copy of your own FBI file, if you have one. Under the Freedom of Information Act (FOIA), the FBI must provide a copy of your file to you if you file a formal request. Be aware that due to security concerns, your file may have portions which are redacted. It is also possible to obtain FBI files on deceased family members and people or organizations of historical interest. Filing an FOIA request is free; directions are available on the FBI's website.