Flu (influenza) viruses are divided into three broad categories: influenza A, B or C. Influenza A is the most common type. H1N1 flu is a variety of influenza A.
H1N1 indicates the viral serotype; It's a kind of shorthand for characteristics that identify the virus to your immune system and allow the virus to enter your cells. There are many different strains of H1N1 flu.
Each year's flu vaccine includes varieties of influenza A — both H1N1 and H3N2 — and influenza B.