svn log & diff, how do they work?

Hello,
I'd like to develop a model of reputation about management of document
diffusion in an organization in my internship. I'm very interested in
2 functions log & diff of subversion. But i don't understand exactly
how do they work? How does the subversion manage different versions ?
How does it detect the modifications of files? Coud you explain them
to me?
and would you send me the code source of these implementations?
Thanks a lot
Best regards

--
Cam Tu PHAN LE
TELECOM Bretagne
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