Sure modern JavaScript libraries have made our lives so much easier, but adding 50K or so can slow down the loading of your pages. Sure after the first load it will be cached the visitor’s browser, but there a are several benefits in using the versions hosted by Google’s content distribution network .
Benefits
Faster loading: I’m sure I don’t have to convince anyone that Google’s infrastructure is hard to beat, since its on their distributed network this means that the library will be loaded from a server near your visitors. Since most browser limit the concurent connections to the same domain, since this file is loaded off a 3rd party server it won’t count toward the browser’s limit for your domain.
Shared Cache: The more people use the service the higher the chance your visitors will already have a cached version from some other website.
Save Bandwidth: Well since Google is serving the file you don’t have to pay for the bandwith.
Up-to-date: Since files are patched and updated with security fixes you don’t have to worry about having a outdated version causing vulnerabilities on your website.
Drawbacks
No source control: you do not have control over the source and that.
Privacy: Since the file is hosted on a 3rd party server, there may be a be a conflict with your site’s privacy policy.
How to use
Couldn’t be simpler just create a script tag pointing to the script on google’s server and your in business.
<script src="http://ajax.googleapis.com/ajax/libs/jquery/1.3.2/jquery.min.js" type="text/javascript></script>
Voilà you’re done, for more details refer to Google’s documentation.