A hypothesis is a statement that contains an explanation to a phenomena. Hypothesis can be either proven or disproven. In most cases the hypothesis is something that can be tested in some way, usually quantitative.

The purpose of the variables is to see how they are related and to allow you to have control over the experiment. The independent variable is what the experimenter alters or enacts in order to conduct the experiment.

The dependent variable is the changes of the independent variable. The dependent variable depends on what happens during the independent variable. In other words, the dependent variable is dependent upon the independent variable.

There are also variables that are called controlled variables. These variables are anything that can have an influence upon the dependent variable.

A hypothesis is your statement of what you think will happen to a dependent variable when a change occurs in the independent variable.

Your dependent variable is the result.  Your independent variable is the causal agent -- the thing that, you think, is making the result happen.  (These are not necessarily the technical definitions -- I figure you can look those up.  I'm trying to tell you how I think it is easiest to think of them...)

So let's say that you have a hypothesis that says that an increase in the size of the police force will lead to a drop in crime.  In that case, the size of the force is the independent variable.  The crime rate is dependent.  You are hypothesizing that the independent variable will have an impact on the depdendent variable.