Gemini: That tag instructs certain proxy servers not to cache anything from a specific page.
It prevents users on a shared Internet connection to view the same (outdated) pages that someone on the same network just browsed.
Mostly used for pages with dynamically inserted data.
As for SE traffic, my Top 5 stats from crawlers/spiders are like this:
Google 85%
AltaVista 8%
Northern Light 5%
MSN Search 2%
These are for a site where nobody bothered submitting to any
engines/crawlers or making any META tags. Many search queries
used to reach the site seems to be a bit awkward (i.e. 15 hits
on the search words "advanced home cooking").
:-)