The U.S. Freedom of Information Act (FOIA) is a law ensuring public access to U.S. government records. FOIA carries a presumption of disclosure; the burden is on the government - not the public - to substantiate why information may not be released. Upon written request, agencies of the United States government are required to disclose those records, unless they can be lawfully withheld from disclosure under one of nine specific exemptions in the FOIA. This right of access is ultimately enforceable in federal court.
This is an issue that made conspiracy theorist happy because it basically gave people access to government documents to let them know what the government is doing.
The nine exemptions to the FOIA address issues of sensitivity and personal rights. They are (as listed in Title 5 of the United States Code, section 552):

(A) specifically authorized under criteria established by an Executive order to be kept secret in the interest of national defense or foreign policy and (B) are in fact properly classified pursuant to such Executive order;
related solely to the internal personnel rules and practices of an agency;
specifically exempted from disclosure by statute (other than section 552b of this title), provided that such statute (A) requires that the matters be withheld from the public in such a manner as to leave no discretion on the issue, or (B) establishes particular criteria for withholding or refers to particular types of matters to be withheld;
trade secrets and commercial or financial information obtained from a person and privileged or confidential;
inter-agency or intra-agency memoranda or letters which would not be available by law to a party other than an agency in litigation with the agency;
personnel and medical files and similar files the disclosure of which would constitute a clearly unwarranted invasion of personal privacy;
records or information compiled for law enforcement purposes, but only to the extent that the production of such law enforcement records or information (A) could reasonably be expected to interfere with enforcement proceedings, (B) would deprive a person of a right to a fair trial or an impartial adjudication, (C) could reasonably be expected to constitute an unwarranted invasion of personal privacy, (D) could reasonably be expected to disclose the identity of a confidential source, including a State, local, or foreign agency or authority or any private institution which furnished information on a confidential basis, and, in the case of a record or information compiled by a criminal law enforcement authority in the course of a criminal investigation or by an agency conducting a lawful national security intelligence investigation, information furnished by a confidential source, (E) would disclose techniques and procedures for law enforcement investigations or prosecutions, or would disclose guidelines for law enforcement investigations or prosecutions if such disclosure could reasonably be expected to risk circumvention of the law, or (F) could reasonably be expected to endanger the life or physical safety of any individual;
contained in or related to examination, operating, or condition reports prepared by, on behalf of, or for the use of an agency responsible for the regulation or supervision of financial institutions; or
Geological and geophysical information and data, including maps, concerning wells.
Other than that it’s open season for information.