Using Google to host your jQuery (or other applicable content) has several advantages over hosting a version on your own server including better caching, decreased latency, and increased parallelism. There are plenty of other articles to discuss the merits of decreased latency benefits of a CDN or the effects of parallelism on browsers so it wont be covered here.
Caching
Perhaps the most compelling reason to use Google to host your jQuery is that your users actually may not need to download it at all. If you’re hosting jQuery locally then your users must download a copy at least once. Your users probably already have multiple identical copies of jQuery in their browsers cache from other sites, but those copies are ignored when they visit your site.
When a browser sees references to a CDN-hosted copy of jQuery it understands they all refer to exactly same file. The browser trusts that those files are identical and wont re-request the file if it’s already cached.
Execution
So, now that we know that Google is a good place to serve up our jQuery from how are we going to do it? I believe the best way is also the simplest:
<script src="//ajax.googleapis.com/ajax/libs/jquery/1.7.1/jquery.min.js"></script>
<script>window.jQuery || document.write('<script src="js/libs/jquery-1.7.1.min.js"><\/script>')</script>
This will point to Google to host the jQuery library, but fall back to local hosting in the event of connectivity issues.