Each year the CDC determines which strains of the flu are likely to be the most prevalent, and creates a flu vaccine to protect against those strains. The vaccine is made of inactive or weakened (attenuated) flu antigens. You cannot get the flu from the flu vaccine, though some mild side effects are possible. The CDC assures us that the side effects are much less severe than the symptoms caused by the actual flu virus.
While everyone should get a flu vaccine every year, it is especially important for children under five years old (and especially under two,) pregnant women, people over 65 years old, people with certain medical conditions including diabetes, asthma, and chronic lung disease, and healthcare personnel or people who live with or take care of those in a high-risk group.
In addition to a greatly reduced chance of getting the flu yourself, there are other benefits to getting a flu vaccine every year: