The word "people" is used in (at least) two different ways: 1. as the plural of the word "person" (this is the most common usage); 2. as a word meaning something similar to a "nation" or "race" or "folk". In this case the plural is "peoples".

As Richard says, the word "peoples" has a very specific use in the English language. If you're ever unsure of which one to use, and feel unsure if this specific use applies, just use the word "people". Most native speakers of English do this anyway. Hope this helps :)

"Peoples" can only be used grammatically when referring to a group of people who are culturally or historically related and form a unique and singular identity. Examples: I have studied the peoples of Sub-Saharan region of Africa. Never: There are many peoples there.

Thank you all for your responses :)