Think of a TFR as a no-fly zone for most aircraft. They temporarily prohibit flight over certain areas, such as sports stadiums and areas of disaster relief or firefighting operations. The FAA may designate a TFR to protect the president and vice president, or to prevent sightseeing or other aircraft from congregating in a congested and unsafe fashion over an event or site that could attract a lot of interest.
With a few exceptions, TFRs generally are short-term and aren't depicted on aeronautical charts. Instead, they are disseminated through notices to airmen (notams), which will tell you where a TFR is, how big it is, and how long it's expected to be in place.
Call Flight Service for a thorough preflight briefing before every flight - even out to the practice area. Ask for all notams and make sure that you understand the dimensions of any TFR near or along your route of flight.
TFRs are more common in some parts of the nation than in others, but if you don't do your homework before a flight, you could fly into one - with scary consequences. You could be intercepted by a military aircraft and required to land, and when you emerge from your aircraft you most certainly will be greeted by law enforcement officials.
A database of selected flight restrictions and notams, including graphics where available and plain-language guidance, is available on AOPA Online. Remember to check with Flight Service and/or DUATS for up-to-the-minute status of all TFRs and notams. Know what to do if a military aircraft intercepts your airplane. (You'll find intercept procedures in the Aeronautical Information Manual Section 6, 5-6-2, and at AOPA Online. For more on U.S. airspace and TFRs, see the AOPA Air Safety Foundation's newly updated online course "Operation Airspace".