A press pool is a group of news gathering organizations that combine their resources in the collection of news. A pool feed is then distributed to members of the broadcast pool who are free to edit it or use it as they see necessary. In the case of print reporters, a written pool report is distributed to all members. A pool spray is a brief photo opportunity, for instance, at the White House following a meeting.
Pool coverage is sometimes required because of the nature of the news event being covered. For example, judges will often allow only one broadcast camera in the courtroom during a trial. As a result, interested broadcasters will select one of their own to provide the "pool camera" for the day, with that organization agreeing to share the footage with all other broadcasters in the pool.
Similarly, a daily rotating pool is set up at the White House for organizations that cover the president. They are allowed special access to certain events with the understanding that the information and footage will then be shared with the rest of the White House press corps. For instance, when the president meets with a foreign dignitary, the two will sometimes make a statement together in the Oval Office. Since it is not possible to accommodate all interested journalists in that room, pool coverage is the only practical way to handle the event.
At other times, pool coverage takes place as a more cost-effective way of covering events that only occasionally "make news." For example, the major networks rely on a pool feed of the daily briefing at the State Department for their live coverage, since the press conference is usually only of national interest when there is a significant breaking story relating to international diplomacy. Since "coverage" of such an event consists merely of pointing a camera at a person standing behind a lectern, a pool feed is a practical means of ensuring the event will be available as needed without all five major US TV news outlets paying for their own individual live crews.
Pool situations are also often set up because of the physical immensity of certain events. One prominent example of this was the international broadcast of the state funeral of John F. Kennedy, which involved four major venues and a lengthy procession. Likewise, presidential inaugurations and political conventions tend to involve pool coverage in the US (particularly since there is no state broadcaster that would naturally handle the coverage on its own).