There are many categories, these are the ones I learned about:
Depressants, Stimulants, Narcotics,
Depressants: Sleeping Pills (Ambien, Melatonin, etc.), alcohol,
They slow down brain function and speed, making normal everyday activities difficult (when under the influence or depressants). If you take the over prescribed amount of sleeping pills and force yourself to stay awake, your brain processing become so slow that you begin to hallucinate.
Stimulants: Adderall, Ritalin, Coffee (Caffeine), Speed,
They speed up brain speed and function, making time feel very slow, because you brain speed is faster than what you are used to. Stimulants help with disorders like ADHD/ADD, improving focus and concentration. But, be wary, if you take one too many pills, you'll be up all night. Take two, and you'll be dead.
Narcotics: Xanax, Vicodin,
They slightly disable the pain receptors at the base of the skull, lowering pain levels in correlation with the dosage. Some say, "I feel high..." Truth be told, you are not high, it's just the feeling of no pain is a new sensation! Sorry, guys...
Now, don't be stupid, don't abuse prescription drugs.