Once a russian explained it to me like this when I made the mistake: «Многие люди» => answers the question "who ? by whom ?, etc" «много людей» => answers the question "how much ? / how many ?, etc" I don't know if it's a correct explaination, but I stick to it.

МНОГО is either: 1) an adverb answering the question ''to what extent?'': много работать, много учиться (the opposite is ''мало") 2) numeral or numeral adjective (I'm not sure about a specific term for it) answering the question "how much ? / how many?'' В комнате было много людей -- There were a lot of people in the room (How many? A lot) МНОГИЕ is an adjective used to describe a large amount or number of something (both people or objects) многие люди знают, что… -- Many people know that... (= Большое количество людей знает, что… -- A large number of people knows that... ) The opposite of it is: Немногие люди знают, что… -- A few people know that...

We use "много" to say that there is generally big amount of something. A lot of people = "много людей". We use "многие" to say that something happens often throughout some set of things, and especially people. Or something is true for the most part of some set. Usually it's used as shorter form of "много" when the actual noun is obvious. "A lot of people have been already drunk" - "многие (люди) были уже пьяны". "It's hard time for the schools. Many of them has closed" - "Сейчас трудное время для школ. Многие из них закрылись". Sometimes it's more idiomatic to use "многие", like in: "It is said by many people" - "многие говорят".