Barbiturates and benzodiazepines.

Compare and contrast barbiturates and benzodiazepines. Why are they prescribed? How long should they be taken, what are the side effects, what is contra-indicated (are there things that should not be done when on them)? What is the difference between drugs that sedate and drugs that are hypnotics? What is the difference between long-acting and short-acting barbiturates and benzodiazepines? Why is the withdrawal from these drugs so dangerous?