Nina wrote:
Hi. What did you do? Pretend I'm, like, eight and/or stupid (because on this I are pretty stoopid).
Once upon a time, websites were written and maintained by individuals, or by relatively small, closely-knit groups of people.
Once upon a time, and indeed even to this day, there has always been a need to try to figure out how "good" a website is. Now, of course, "goodness" is a terribly subjective and multifaceted concept, so trying to reduce it to a unidimensional metric or "rank" is a task fraught with peril and ultimately utterly impossible, but the need is strong enough that people are bound to try anyway. One area in which the need is real is: ranking search results. It's (comparatively) very easy to write a web search engine that returns links to every single page where a user's search terms are mentioned. It's much, much more difficult, however, to rig it up so that the user can easily zero in on the *interesting* or *useful* links first, without having to wade through all of the hundreds or thousands or millions of hits which a simpleminded brute-force search engine might yield.
Once upon a time, Larry Page and Sergey Brin had a great idea. They were trying to get a handle on "goodness" as defined by the *users* of a page, *not* on the goodness that the authors of a page wished it had, or might try to assert that it had. Even more to the point: Larry and Sergey wanted to rank "goodness" in terms useful to the people doing the searching, not in terms useful to the owners of the websites where the hits might (or might not) be found. L&S realized that one way to get a handle on this user-perceived goodness was to look at how many people linked to a given page. Simply speaking, to first order, the more people link to webpage X, the "better" webpage X is, and the higher webpage X should appear in a list of search results.
As an additional, second-order wrinkle, the founders of Google realized that not all links are created equal. Among other things, links to an unknown site *from* sites that are known to be "good" count more towards ranking site X's "goodness" than do links from other, random, unknown or not-so-good sites.
Needless to say, this strategy turned out to work very, very well. Google is now very, very successful, and its name has become literally synonymous with "to do a web search for".
However, fast-forward to today. There are now "social" websites which are most assuredly *not* written and maintained by "individuals or relatively small, closely-knit groups of people". Social websites, such as blogs and wikis, are by definition written and maintained by anybody and everybody out there on the whole world-wide internet. And that's a fine, wonderful, libertarian and egalitarian thing -- except that it collides head-on with Google's strategy. The collision wouldn't matter so much if there weren't social websites with high pagerank, or if Google and its pagerank algorithm weren't so successful. But in a world where Google is far and away the #1 search engine, and where the ever-so-social (or at least ever-so-wiki) site known as Wikipedia is a top-10 website with stupefyingly high pagerank, we have the makings of quite a fine little quandary.
Simply put, Wikipedia is an absolutely irresistible, boron-neodymium supermagnet for linkspam. The World Wide Web is no longer Tim Berners-Lee's theoretically interesting research lab thingy, it's an unignorable real-world phenomenon. If you're a commercial website operator, having high Google pagerank is money in the bank. So if you yourself can go in and create links from a high-pagerank site like Wikipedia to your grotty little commercial site, well, you'd be a fool not to.
So Google, like virtually all wildly successful and unignorable real-world phenomena, has had to compromise a bit on its principles. Links from high-rank sites contribute higher to a linked-to site's pagerank, *unless* the high-rank site is openly editable by anybody, in which case the links probably have to be ignored. So Google invented the new HTML link attribute "nofollow". (One of the nice things about open, extensible languages like HTML is that anybody can invent new extensions like this to it anytime.) Nofollow means, "don't weight this link by my site's pagerank when totting up the linked-to website's pagerank", or indeed, "don't follow this link in order to tot up pagerank at all". Google invented this attribute specifically for high-pagerank social sites such as Wikipedia, and encourages us to use it on our user-editable external links.
In terms of "what did specifically Brion do to turn on the "nofollow" attribute for Wikipedia external links?", that I can't answer. Probably some propellorhead computer weenie thing involving "property lists" or "php configuration variables" or suchlike. :-)