If there is a statistical connection between two parameters of data such that a change in one is associated with a change in the other, they are said to be correlated. For instance, from childhood until adulthood, there is a correlation between age and height; older people are generally taller and taller people are generally older.
If two quantities correlate in such a way that an increase of one quantity is associated with an increase in the other, they are said to be positively correlated. Likewise, an increase in one quantity associated with a decrease in the other is called a negative correlation.
The more the data points appear to follow a specific trend, the more correlated they are. If they are situated almost exactly on a line, the quantities are said to be strongly correlated, while if they are more spread out, the quantities are weakly correlated.