Why doesn’t my site description update in Google and other search engines as soon as I update my site?
Google and other search engines show visitors a preview, or “snippet”, of each site in the search results:
If you make changes to your site, the preview doesn’t update right away. That’s because the search engines generate these previews from their own “cached” copies of your pages on their own server disks: they’re not fetching a live copy of your site at the time they show people the search results.
The preview won’t change until the next time they update their cached copy by re-indexing your pages. The outdated preview will usually fix itself within a few days.
Google explains more about this, and offers a (complicated) way to remove the preview snippet more quickly if it’s (for example) causing legal problems, on their Webmaster Central Blog.