What causes influenza?

After reading the article from  https://www.cdc.gov/flu/prevent/vaccinations.htm

• What causes influenza?
• What are some of the impacts of the influenza viruses on the body?
• What is the difference between influenza and the common cold?
• How does the flu vaccine work?
• Who should get vaccinated? Who should not get vaccinated?
• Why are some people in support of vaccinations and others not?