Usually, we are looking to see if there is a relationship between the two sets of data. We draw a line of best fit. This should have roughly the same number of points above and below it.
The less scatter there is about the best-fit line, the stronger the relationship is between the two quantities. If the points are close to the best-fit line, we say that there is a strong correlation. If the points are loosely scattered, there is a weak correlation. We say there is zero correlation if there is no linear relationship between the variables- in other words if we can't draw a meaningful best fit line. - See more at: http://www.mathsrevision.net/gcse-maths-revision/statistics-handling-data/representing-data#sthash.HYcOfatU.dpuf
Read more at http://www.mathsrevision.net/gcse-maths-revision/statistics-handling-data/representing-data#0SgJwt7xhLIdX6Vm.99