Correlation is a statistic that measures the degree to which two variables move in relation to each other. Correlation measures association, but does not tell you if x causes y or vice versa, or if the association is caused by some third (perhaps unseen) factor.
It ranges from -1.0 to +1.0. The closer r is to +1 or -1, the more closely the two variables are related.
+1.0 = Strong Correlation in same direction
-1.0 = Strong Correlation in opposite direction
0 = No relationship between the variables.