When you follow a link on the web, your browser sends a request (for a particular web page) to the server at the new site you're going to. That request includes, among other things the URL of the page the link was on, which is known as the "referrer" URL. (Or sometimes "referer", because someone in the early days of the web misspelled it.)
One of the interesting things to me about the web is that the owner of a given site can sometimes (depending on their setup) view a list of the URLs that visitors came from. (Note that this only works if people actually follow the links; just having a link in place doesn't notify the other site that the link exists.) Which means they can go see what other sites are saying about them, and sometimes can even post comments at those referring sites.
This fact makes me a little nervous sometimes when I post links to other people's sites; they might follow the referrer URL and find out what I said about them! On the other hand, knowing that makes me generally think a little about whether the owner of the site I'm pointing to is likely to be unhappy with what I'm saying, and if so, whether I want to be more careful about what I'm saying.
Even so, I'm sometimes surprised (but usually pleased) when someone I link to stops by here and posts a comment on the entry in which I linked to them. I like it; I think it adds to my sense of there being an ongoing conversation, makes me more aware that sites are put up by real live humans.
I've been thinking about this stuff for a while, but the proximate cause of my posting this today was getting a comment on my previous entry from Ed Fitzgerald of Unfutz, noting that the latest version of his electoral college survey had been posted. I'd been planning to stop by his site today and see if this week's edition was up, but he saved me the trouble. So, thanks for the note!
Oh, and this week's survey includes a cool new graph, underneath the old one, showing all the data points rather than just aggregates of them.