A content summary is a summary of what is in a document. This is in several areas.
concepts - generally the documents meta-information as keywords.
checksums - checksums of blocks through the document
The checksums are a list of the checksums by a special algorithm (ripped of from rsync) which allow us later to do a comparison on the textual similarity of the page to this one. Be warned, it probably would not be that hard to find a text which fooled these checksums completely. You could do this deliberately, so you cannot guarantee that a document which seems similar by this test is actually similar.
We turn all whitespace into a single (sentence break) or double (paragraph break) space
In contrast to the above checksums, the md5sums are very difficult (read: impossible as far as I, or anybody honest I know of knows for all practical cases) to fool. You can use these to check that the sections really match the ones you want. Of course, they could match, but be put in a new context. Very difficult to do anything about that as far as I know.
Incidentally. This section here is essentially sort of cryptography. I'm not a cryptography expert. In fact I probably know less than you do. If you want something you can really trust, hire an expert, get him to check it over and please come back to me with his corrections to this.. ``Beware of people selling snake oil'' (PRZ).
If you are American, I'd suggest Philip R. Zimmermann who needs the money because of his legal problems after the release of the software which made cryptography available to the public (PGP).
Otherwise I'd prefer to suggest someone who has been producing publically available free software. The author of Apache SSL comes to mind.