[Yum] about yum option- update and upgrade?

[SB CH]

Hello, all.

I'm a beginner of the yum.and I have some question.

* What is the difference between yum "update" and "upgrade"?
   Surely, I saw man, but I can't understand.
   If I execute yum upgrade at redhat 7.3, then my system 
   is upgraded with 9.0?

* When I execute "yum update", is ther any  dependencies problem?
  If some dependencies problem occurs, the system will be unstable. 


Thanks in advance.

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