People, they (in general): người ta

Người ta (literally "people") is the general "they" or "people": Người ta nói tiếng Việt khó lắm. People say Vietnamese is really hard. Use it for what people usually do, or what's said, without naming anyone: Ở đây người ta ăn sáng sớm lắm. People here have breakfast really early.

It also covers "they" or "someone" when you don't know or don't care who: Người ta đang sửa đường. They're fixing the road. Where English uses a general "you" ("you don't do that here"), Vietnamese often uses người ta, or leaves the subject out: Ở đây người ta không làm vậy.

Họ is "they" for a group you could point to: those tourists, that company. For people in general, người ta is the natural word.

In casual speech, người ta can also mean one particular person you'd rather not name: Người ta có bạn gái rồi. He's already got a girlfriend. Between friends and couples it can even mean "me", said with a pout: Người ta giận rồi đó! I'm cross now! The tone of voice makes this clear.