Difference of 'mtime' and 'ctime'

[Guangliang He]

In the man page of 'find' command, you can find the following two options,

          -mtime n        True if the file has been modified in n
                          days.

          -ctime n        True if the file has been changed in n days.

Can any one out there tell the difference of these two options? 

Thanks in advance.
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