Direct and Inverse Relationships

Let us say that we are considering two quantities which we can call x and y. These could be anything - temperature and pressure, population and crime rate, weight and height, etc. If these have a direct relationship (or a direct correlation as we might also call it) that means that if you increase one, you increase the other, or if you decrease one, you decrease the other. So if x increases, y also increases. If it is an inverse relationship (or correlation) that means that increasing one will decrease the other. If you get more x, you will then have less y.
Note that there is a third alternative. Many things have no relationship, either direct or inverse, so if you increase x, that tells you nothing about what will happen with y. These quantities would be said to have no correlation.