What are the Classification of drugs according to usage?

This question is too vague. There are various types of drugs. If you are referring to drugs related to psych, then there are SSRIs (selective serotonin reuptake inhibitors), MAOIs (monoamine oxidase inhibitors), antipsychotics, TCA (tricyclic antidepressant), there are a lot. And typing down their use and effects one by one would be too long. Wikipedia or google might be of assistance.