"пить" simply means "to drink" in two variants:
1) In Present Simple Tense for regular meaning:
Я пью молоко. - I drink milk. (with the meaning "I can use milk as food" or "I drink milk from time to time")
Я пью молоко каждый день. - I drink milk every day.
2) In Present Continuous:
Я пью молоко. - I am drinking milk.
"выпивать" has at least two meanings:
1) to drink up, i.e. to drink all the drink:
Выпей молоко! - Drink up the milk!
Sometimes the meaning is really close to "пить":
Я выпиваю один стакан (или два стакана) молока в день. = Я пью по одному стакану (или по два стакана) молока в день. - I drink one glass of milk a day.
2) Without specification of what is drunk it means "to drink alcohol from time to time".
Oh! By the way :) "пить" without specification can mean "to drink alcohol" and in some cases "to drink alcohol in large amounts" :)