.3 (positive weak)

-.9 (negative strong)

-.3 (negative weak)

.9 (positive strong)

One variable causes the other.

There is no relationship between the two variables.

There is a positive relationship between the two variables.

The variables are dependent on one another.

Examining the relationship between temperature and ice cream sales

Exploring whether there is a relationship between shoe size and height

Testing whether people who vote are more likely to own a home

Analyzing if time spent studying is related to test scores

When the relationship between variables is curvilinear. 

When your explanatory variable is quantitative and your outcome is categorical.  

When your explanatory variable is categorical and your outcome variable is quantitative. 

When your explanatory and outcome variables are both quantitative.