Я согласен с Shady_arc. Добавлю немного примеров:

Каждый день он пил молоко. - Everyday he drank milk (or he used to dring milk every day). - Habitual action, imperfective.
Он пил молоко и читал газету. - He was drinking milk and reading a newspaper. - Continuous action, imperfective.
Он выпил молока и вышел из дома. - He drank some milk and left home. - Completed consecutive actions, perfective.

А вот примеры, как это связано с алкоголем:
Я хочу пить. - I want to drink. I am thirsty. (Unspecified)
Я хочу выпить чаю. - I want to drink some tea. (Specified. Limited amount assumed - so, it's perfective).
Я хочу выпить. - I want to drink some alcohol. (If the context does not suggest anything specific, than it usually means alcohol).