NTFS permissions for files

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While going through MS Press books for 70-271, i had a doubt concerning NTFS permissions for files.

Read
User can read the contents of the file and view the properties of the file, including permissions, ownership, and file attributes

Write
User can overwrite the file entirely (but not modify its contents), change file attributes, and view permissions and file ownership

What is the difference between overwriting, modifying the contents of the file and reading the contents of file?