what's the different with hi, hello, hey, yes, yeah, yah, yep?
Does they have same meaning? Does they only different emotion？

Question

what's the different with hi, hello, hey,   yes, yeah, yah, yep?
Does they have same meaning? Does they only different emotion？

Answer

Ranked by level of formality (in my opinion)

Greetings:
1. Hello
2. Hi
3. Hey

Confirmations:
1. Yes
2. Yeah
3. Yep
4. Yah

Although "Yah" actually sounds more like German or something other than English.

Hi and Hey are also somewhat tied. They often have somewhat different tones.

Hey can mean hello or it can mean "I want your attention."

I live in the Southeastern US. A common local phrase in this part of the US is "hey y'all." This is somewhat informal, but it isn't rude at all. It means "Hello, group of two or more people."

Answer

The meanings are the same but they are different in degree of formality.
"Hi" and "Hello" are a bit more formal compared to "Hey".
The same story is true about "yes" and "yeah" compared to "yep"