If like me you ever wondered about the differences between those different types of standard (usually employed for webfeeds), here is a definition by Ben Hammersley taken from his slides at ETeach 2005.

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Simple, adhoc, temporary, loosely defined data, very loosely defined standard, many many uses Fantastic for machine readable lists. Useless for anything else Complex, strict, preplanned, strongly defined data, strongly defined standard, burdened with evil buzzword. Fantastic for complex document mining. A nightmare for tiny ad-hoc apps. Simple, strictly defined data, strictly defined standard, with extra architectural loving

Check his tutorial slides (pdf), it's definetely worth reading!