The Freedom of Information Act makes it mandatory for a public authority to disclose whether or not it has the information requested of it. The same is also required of privatised bodies (such as utilities companies), police and fire authorities and quangos.

Exciting piece of information: Quango is an acronym for "QUasi-Autonomous Non-Governmental Organisation."

There are some exemptions, such as security and intelligence services, Special Air Service, Special Boat Service, and GCHQ (never ask for information from someone who loves in a doughnut). Chris also mentioned NCIS (National Crime intelligence Service). It's worth mentioning that this organisation ceased to exist in 2006, when it was merged into SOCA (Serious Organised Crime Agency), amusingly on April Fools' Day. 

Information must be released unless it is judged that the public interest is not served, or if the public interest in NOT releasing the information is greater than in releasing it. See PCC Code of Conduct if you're not sure what public interest means.

The body from which the request is made must respond within 20 days:
"Do you have this information?" - 20 days
"Can I have it?" - another 20 days

What is "information"?
The act specifies that only information that is recorded or written down can be requested. "Sofa conversations" don't count. None of the information about the initiating of the war in Iraq was written down, so it remains largely secret.  Nice to know that the decision to go to war was made on a Chesterfield over a cup of tea and some French fancies.

Section 16 of the act says they must assist you and must not mislead you, but they can charge you a "reasonable" price and can also refuse information on the basis of "prohibitive expense".