What is H1N1?

I heard of it, but I'm not sure what it is.

Influenza A (H1N1) virus is a subtype of influenza A virus and the most common cause of influenza (flu) in humans. Some strains of H1N1 are endemic in humans and cause a small fraction of all influenza-like illness and a small fraction of all seasonal influenza. H1N1 strains caused a few percent all human flu infections in 2004-2005. Other strains of H1N1 are endemic in pigs (swine influenza) and in birds (avian influenza).

Sites for Reference:
http://en.wikipedia.org/wiki/H1N1
http://www.who.int/csr/disease/swineflu/en/
http://www.cdc.gov/H1N1FLU/