Some WordPress plugins also add cache functionality to your blog. This helps your blog load faster because WordPress can retrieve the pages of your blog from the cache instead of generating them all over again.
All good cache plugins will clear the cache when a post, page, or comment is published. However, if you make other changes (e.g. to your theme), the cache may not be cleared and the old version may still appear. In this case, check the plugin's instructions to find out how to clear its cache.
Note that WordPress does not come with a cache by default, so the above would only apply if you installed a cache plugin yourself.