A pandemic happens when a completely new strain of flu virus develops which no-one has built up any immunity against. As a result, the new flu strain spreads very rapidly around the world and affects many people. This has been seen recently with the outbreak of swine flu.
A pandemic could start at any time of the year. Existing vaccines will not protect against the new strain and new vaccines take time to develop, and so are not available immediately. The symptoms of a pandemic flu strain are likely to be similar to seasonal flu but may be more severe and cause more complications.
Historically, pandemic flu outbreaks have happened every few decades. The 2009 Swine flu outbreak was an example of a relatively mild pandemic. Health organisations in the UK and around the world are closely monitoring flu viruses to anticipate a pandemic, and very detailed plans are in place to help people to respond if and when a pandemic happens.