Quick Answer
Alles means "everything", but "alle" means "every" or "all". When you want to use "alle", you need to be able to count the items: Alle Hunde, alle Pflanzen, alle Freunde.
Sometimes you hear beginners ask about the difference between "alle" & "alles" in German. Telling the difference can be especially tricky when it looks like the two words could be swapped out for one another:
Er hat alles gehört.
Er hat alle gehört.
However, alle and alles are not in fact interchangable. They have different meanings.
The difference between alle & alles is very similar to the difference between "all" and "everything" in English.
Whenever you would use "everything" in English, you'd say "alles" in German.
So the first example sentence means, that he has heard everything. Whereas the second example sentence means that he has heard all (of them) or everyone.
Das ist alles der gleiche Preis.
Ich habe gestern alles geputzt.
Bezahlst du alles auf einmal?
Haben alle Getränke den gleichen Preis?
Ich habe gestern alle Zimmer geputzt.
Bezahlst du alle Rechnungen auf einmal?