CSOM help

Hi All,

I am new to CSOM, I am working on few simple operations in the CSOM. Please clarify me on the below points.

1) How CSOM will differ from server side code execution
2) what is difference between Load() & ExecuteQuery()
3) I have dev & qa servers, In the dev server I am trying to do some CRUD operations on the qa site, but it is throwing the below error though I gave sitecoll admin permissions to the dev user.

Access denied. You do not have permission to perform this action or access this resource csom.

Can you please guide me the correct way & let me know if you have any queries.

Thanks In Advance,
Kumar.