What is the difference between a flu and a cold?

Usually severity: Both are caused by viruses. Generally the respiratory flu is associated with fever, body aches, cough, chills and extreme fatigue. A cold is usually much milder with congestion, runny nose, mild cough.

Flu or cold?: A cold is a milder respiratory illness than the flu. While cold symptoms can make you feel bad for a few days, flu symptoms can make you feel quite ill for a few days to weeks. The flu can also result in serious health problems such as pneumonia and hospitalizations. See more infö here: http://www.webmd.com/cold-and-flu/cold-guide/flu-cold-symptoms

Flu & cold : Cold is a milder illness whereas flu on the other hand is more severe form of cold which can result in complications like pneumonia.

Real flu = influenza: Non-medical people commonly ( and inaccurately) use the word 'flu' to describe feeling unwell with a virus. The viruses most people are talking about can make us feel quite miserable. But true influenza is a terrible illness that really knocks you around and can make you feel extremely unwell for days-weeks. Usually with very high fever, cough, sore throat, severe muscle aches and pains.

None: Really they are lay terms for describing a viral syndrome.