Because cold and flu symptoms are quite similar, it's easy to mistake one for the other. In general, colds are milder than the flu. People with colds often have stuffy, runny noses, whereas people with the flu do not. Symptoms such as fever, body aches, extreme tiredness, and dry cough are more common and intense with the flu. Because cold and flu symptoms are so similar, a misdiagnosis could mean serious complications get overlooked if the flu is mistaken for a bad cold.
(To learn more about this topic, click Cold and Flu Symptoms. This article goes into greater detail about recognizing the different symptoms and also provides a chart for a side-to-side comparison.)