In the news we've heard of a number of different types of the flu: the "swine" flu, the "avian" flu, the "spanish" flu, etc.
Strains of flu are designated based on the h portion and the n portion, two genetically determined units of the influenza virus
(ex h1n1 is the "swine flu
"). These can mix and match to make new strains, or alter entirely. The flu vaccine has 3 common strains for that year.