Flu is a disease caused by the influenza virus, a highly mutant DNA virus. Due to the high mutation rate of the virus, that forms many different strains, flu always presents epidemic features in affected populations and people may have several flu episodes during life (the immune response made from previous infections is not efficient in future infections).
The vaccine against flu is a vaccine made of attenuated virus of three different strains. Each year the WHO (World Health Organization) researches and determines which are the strains that should compose the vaccine. This is a strategy to face the high mutation rate of the virus.