The variables are dependent on one another.

One variable causes the other.

There is no relationship between the two variables.

There is a positive relationship between the two variables.

.3 (positive weak)

-.3 (negative weak)

.9 (positive strong)

-.9 (negative strong)

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

Exploring whether there is a relationship between shoe size and height

Analyzing if time spent studying is related to test scores

Examining the relationship between temperature and ice cream sales

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

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

When the relationship between variables is curvilinear. 

When your explanatory and outcome variables are both quantitative.