Warning: The guest page cache system will cause those not logged in to not see a current version of your site. It is therefore only useful for sites where the content hardly ever changes.
The guest page cache system, which you can turn on at Admin Panel -> Miscellaneous -> Advanced Options, allows your pages to load faster by caching a copy of what the page looks like. This means that you should only use the cache if there's a single way you want each page to look to all people who aren't logged in. If you have multiple template sets or multiple languages you want to let people choose among, you will not want to use the cache. Keep in mind that WSN already caches some slow queries and toplists automatically and you should only consider this cache system if your pages are too slow or server load too high.
If your site doesn't get a lot of non-guest traffic, you'll need to set up a helper cron to ensure that timed tasks still all get performed.
After submissions are made it automatically updates the cache of the destination page, but it cannot update any pages that for example might include a toplist which does or should reference the updated/new page. Please note that the cache can use up huge amounts of disk space on a large site. If you have 500 forums and 15,000 topics, it could take a gigabyte to fully cache. The total cache size will be bytes-per-page * number-of-pages * number-of-usergroups-cached. If your space is limited you can set a limit on the total size in your cache settings, but the smaller you make this value the more often slow uncached pages will be served.
After making template changes or the like you may wish to reset the cache, by clicking the "empty cache now" link on the advanced options page.
The cache system excludes the admin panel, plus anything under these files on the user-side: edit.php profile.php privatemessaging.php buddylist.php online.php 2checkout.php paypal.php nochex.php activate.php upgrade.php setup.php attachimport.php chat.php im.php in.php modcp.php newposts.php savedtopics.php searchips.php securityimage.php revisepost.php addpoll.php invites.php