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guo huaguo hua 

What is the difference and relationship between these document sites?

When I asked a question, I've got the answer like this:
Please follow the below link. It will be helpful for you.
1.) http://www.salesforcetutorial.com/create-salesforce-application/
2.) https://help.salesforce.com/articleView?id=dev_tabsetcreate.htm&type=0
3.) https://ccoenraets.github.io/salesforce1-app-tutorial/create-expenses-tab.html

What is the difference and relationship between these document sites?
http://www.salesforcetutorial.com
https://help.salesforce.com
https://ccoenraets.github.io

Should I read them all? Are there any repeated content, that will take a lot of my time to read?

Thanks!