Swine flu, also known as 2009 H1N1 type A influenza, is a human disease. People get the disease from other people, not from pigs. The disease originally was nicknamed swine flu because the virus that causes the disease originally jumped to humans from the live pigs in which it evolved. The virus is a "reassortant" -- a mix of genes from swine, bird, and human flu viruses. Scientists are still arguing about what the virus should be called, but most people know it as the H1N1 swine flu virus
The swine flu viruses that usually spread among pigs aren't the same as human flu viruses. Swine flu doesn't often infect people, and the rare human cases that have occurred in the past have mainly affected people who had direct contact with pigs. But the current "swine flu" outbreak is different. It's caused by a new swine flu virus that has changed in ways that allow it to spread from person to person -- among people who haven't had any contact with pigs.
That makes it a human flu virus. To distinguish it from flu viruses that infect mainly pigs and from the seasonal influenza A H1N1 viruses that have been in circulation for many years, the CDC calls the virus "2009 H1N1 virus." Other names include "novel H1N1" or nH1N1, "quadruple assortant H1N1," and "2009 pandemic H1N1."
Many people have at least partial immunity to seasonal H1N1 viruses because they've been infected with or vaccinated against this flu bug. These viruses "drift" genetically, which is why the flu vaccine has to be tweaked from time to time. But the H1N1 swine flu is not the usual "drift variant" of H1N1. It came to humans from a different line of evolution. That means most people have no natural immunity to H1N1 swine flu. The normal seasonal flu shot does not protect against this new virus.
Some people who may have had seasonal H1N1 flu before 1957 might have a little bit of protective immunity against the new virus. That's because seasonal H1N1 flu strains that circulated before 1957 (and which were replaced by the 1957 pandemic flu bug) were genetically closer to the 2009 H1N1 swine flu. This protection is not complete. While relatively few elderly people have had H1N1 swine flu, many of those who did get the disease became severely ill.*