Now this is a question close to my heart because I work with respiratory viruses.
I will assume you refer to flu that is caused by influenza virus. Most people confuse flu with “common cold” that is caused by other viruses such as Respiratory Syncytial Virus (RSV), Rhinovirus, Coronavirus, etc.
Flu is a respiratory infection and is characterized by mainly sneezing, sore throat, fever, runny nose. It is spread mainly through droplets such as sneezing, mucus left on surfaces (e.g. desks, door handles, phones). It is therefore very important to use a handkerchief when you have flu to stop the spread and also wash your hands regularly not to catch it.
Because viruses change very fast, every year scientists develop vaccines to prevent the seasonal flu. However, some influenza viruses can sometimes cross between hosts, e.g. from animals to humans and cause a pandemic where it spreads very fast and several people die!
Remember, the control of spread of flu is very simple: wash your hands regularly with water and soap, use a handkerchief/tissue to cover your mouth/nose when you sneeze, and clean surfaces to get rid of flu viruses. Have a flu free day!