Many people use the word "flu" to refer to any particularly unpleasant infection of the upper respiratory tract, but most of the time they just have a bad cold. Unlike a cold, flu is caused by just one type of virus called influenza.
In North America, flu almost always strikes from October to March. The fact that people gather indoors in winter is partly responsible. Stronger epidemics come every two or three years, infecting twice as many people as during an "off" year. On average, about 30,000 people die each year in North America from complications of influenza. Most of these victims are elderly.
Influenza poses a grave threat to humanity. The most severe worldwide pandemic, the "Spanish Flu" of 1918-19, killed about 30 million people, more than World War I or the Black Death. Influenza is dangerous because the virus mutates very quickly. There's always the possibility that a strain will appear for which no vaccine has been prepared. Some mutations, like the Spanish Flu, can get past the respiratory tract to attack vital organs.