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.