Correlation refers to how a characteristic is common to a group, as in the correlation between hard work and success. Correlation is the mainstay of engineering and science. It is used to determine the relationships between factors (see cause and effect).
When one factor changes, and another factor changes along with it, there is usually a direct relationship. Alternatively, both could be caused by a third factor; for example, two prices of unrelated goods increasing during a period of inflation (see also correlation is not causation).