Direct variation

When two variables increase or decrease together at a constant rate. For example, the amount of money earned increases as the number of hours worked increases. 

Inverse variation

When one variable increases as the other decreases, or vice versa. For example, the amount of time to complete a job decreases as the number of people working on it increases. 

Joint variation

When a variable varies directly with the product of two or more other variables. For example, the area of a triangle is jointly related to both its height and base. 

Combined variation

When a variable depends on two or more variables, either directly, inversely, or a combination of both. For example, the average number of phone calls between two cities is jointly proportional to the populations of the cities, and inversely proportional to the square of the distance between the cities