A pandemic is a new infectious disease that spreads around the world.
The best recent example of a pandemic is AIDS, caused by a virus new to humans: HIV.
Seasonal flu viruses spread around the globe and cause 250,000 to 500,000 deaths each year -- including some 36,000 annual deaths in the U.S. But seasonal flu isn't considered a pandemic, even though the viruses that cause them change a little from season to season.
One of the seasonal flu viruses is a type A H1N1 virus. But the type H1N1 swine flu virus that appeared in 2009 is an entirely different virus. It carries genes from swine flu viruses from North America and Eurasia as well as genes from human and bird flu viruses.
Humans have never before been infected with this virus. That means that nobody is immune, although some people born before 1957 may have been exposed to an ancestor virus that could possibly give them a small degree of protection.
Because the vast majority of people are vulnerable to the 2009 H1N1 swine flu virus, because it spreads easily from person to person, and because the virus is spreading in communities in different parts of the world, the current swine flu has reached pandemic proportions.
Flu pandemics occur regularly. That's because there are many kinds of flu viruses in animals (mostly birds), but so far only a few have evolved the ability to infect humans. There were three flu pandemics in the 20th century: in 1918, in 1957, and in 1968.