nothingmuch is pretty much correct above.
There's a database table called "vote" which associates user IDs and node IDs, so you can keep track of who voted on what. There are additional safeguards in place to keep the voting system from being spoofed by votebots (one of them is an MD5 sum that is sent out, then checked on its way in).
The Voting/Experience System is an add-on to the base Everything system. There's a nodeball for it on the Everything developer site, but I wouldn't swear that the version here hasn't been updated and/or modified. Probably best to ask vroom, who authored it.