Flu (influenza) and COVID-19 are contagious respiratory illnesses, each caused by different viruses. COVID-19 is a result of a coronavirus infection and flu is from influenza viruses. There are other differences between them, such as how they are spread. However, their similarities in symptoms can be confusing, so it's important not to make a judgement on symptoms alone and to get a professional diagnosis.
This is a companion discussion topic for the original article at https://patient.info/news-and-features/how-to-tell-the-difference-between-covid-19-and-flu