Different paradigms are basically different ways of thinking about or looking at problem solving. Programming paradigms usually also include ways of organizing code or design.
The main purpose of any paradigm is to reduce the number of possible solutions to a problem from infinity to a small enough number that you have a chance of picking one. Most proponents of a given paradigm argue strongly that their paradigm of choice removes more possible invalid solutions without seriously impacting valid solutions.
In actual fact, every paradigm eliminates both valid and invalid solutions to any given problem. This is not necessarily bad. However, it does mean that by choosing a particular paradigm, you are closing yourself off from potentially useful solutions or ways of approaching a problem.