When using these feeds, you should cache them whenever possible to speed up the page load of your website, and to reduce the load/bandwidth usage of your site.

JSON (JavaScript Object Notation):
JSON can be easily plugged into any script, since it is formated the same way you would declare the data in javascript code.

You can easily plug this feed into any page, and can assign the data to any variable name by calling the feed with: "?json=[variable name]" query string.
(ex: server.js?json=serverData).

Caching: JSON feeds usually are used client side by browsers, and therefore do not have to be directly cached)

PHP Serialized:
The PHP feed can be easily used in your PHP code by grabbing the feed and calling unserialize() on the data.

Caching: An easy way to cache a php feed is to write the data into a file on the server, and use filemtime() to see whether the cache needs to be updated.
(ie: time()-filemtime($filename) > 60 )

XML (Extensible Markup Language):
XML feeds are the most versatile of any of the feeds here. If you are not going to be using the feed with PHP or Javascript, you will want to use this feed. Though it is a bit more difficult to use, it can be used with any language.