What is correlation?

Correlation is the degree to which one set of data is linked to another (e.g. height and weight.)  Correlation is measured by a correlation coefficient?

How to I read a correlation coefficient?

If the coefficient is between zero and positive one, there is a positive correlation, with +1 being the most reliable correlation.

If the coefficient is between zero and negative one, there is a negative correlation (there is an inverse relationship), with -1 being the most reliable correlation.

A correlation of zero means there is no relationship.

NOTE: Because social events can not be measured as precisely as data collected for the natural sciences, in social sciences, there are seldom correlation coefficients higher than .5 or lower than -.5.  However, these correlations are usually considered significant.

How do I measure correlations?