"alle" is everyone. It's used for people. "alles" is everthing. It's used for things.

Sorry, GoodGirl, that's not quite correct. "alle" - all (of them), i.e. all of the things or people mentioned before; every single one (of the things or people mentioned before), e.g. "Ich suchte im Zug nach einem leeren Abteil, aber alle waren besetzt." - I was looking for an empty compartment on the train, but they were all occupied "Ich sehe meine Freunde selten, aber zu meinem Geburtstag sind alle gekommen." - I don't see my friends very often, but all of them came to see me on my birthday. "alles" - everything "Er macht immer alles kaputt!" - He always ruins everything! "Hast Du alles, was Du brauchst?" - Have you got everything you need?