To answer Mike's comment....
I once set a website up for myself and put a link from it to a number plate related website. My new website within 3 weeks ranked number 4 on google for search term "number plate". This to me implies that Google looks at websites that link to each other and deems them as relevant to each other.
Following on with that theory, if a website has a lot of links to and from it from different sectors websites then I would presume it would take the industry sector with the more volume of links and apply that.
Now, a few months ago there were rumours that Google owned some software that had intelligent enough AI to "understand" a website, and was able to read a website and understand what it was talking about to a degree and understand whether the sentences made sense or were spammy. If they made sense it worked out what search terms would be relative for that website and thus rank it.
Fact ... Google did buy out an AI Technology company, early last year or end 2003 I believe.
There has been dicussion on WebmasterWorld and SEOChat for many many months about the importance of relevant inbound links, and that they carry considerably more weight that "volume" generic links. It is believed to be the aim of Google to remove the element of link farming from SEO.