People who generate syndication feeds have a choice of feed formats. Since 2005, RSS 2.0 and Atom 1.0 are the two most likely choices. RSS and Atom are both a similar technology which was developed to help people receive automatic updates of their favorite websites. Both are wide are widely supported in all major consumer feed readers. RSS has gained widespread distribution through feed reader implementations earlier than Atom, which led to higher popularity.
Although there are some differences between the technology of RSS and Atom, they both do essentially the same job: they fetch updated content from favorite sites in a format that can be easily read in a feed reader. However, Atom has several advantages over RSS, such as less restrictive licensing, IANA registered MIME type, an XML namespace, support for relative URIs, and Relax NG support. Technically, Atom should be considered the more advanced syndication format between the two.
Before RSS and Atom were invented, web users had to bookmark their favorite sites, and return to the bookmarked sites manually to see if anything had been updated. This creates more work for the readers, which means that they could easily miss new content if they forget to chech their bookmarks. It can also be complicated in trying to keep track of many bookmarked sites at the same time
If a person is subscribed to a website's feed, he/she is asking the website to send him/her new content, instead of having to fetch this manually. The person will be sent new website content each time the site is updated.
In its native form, RSS is difficult to read and look a little like raw HTML code. Feed readers translate this into something which is readable, and looks a little like a web page which focuses on written content. Atom has a more carefully-designed payload container.
RSS requires feed-level title, link, and description. RSS does not require that any of the fields of individual items in a feed be present. While Atom requires that both feeds and entries include a title (which may be empty), a unique identifier, and a last-updated timestamp.
History
RSS (originally RDF Site Summary, or Really Simple Syndication) and Atom are members in a family of web feed formats, used to publish frequently updated contents such as blog entries, news headlines, audio, and video in a standardized format.
RDF Site Summary, the first version of RSS, was created by Dan Libby and Ramanathan V. Guha at Netscape in 1999. This version became known as RSS 0.9. In July 1999, the newer version, RSS 0.91, simplified the format by removing RDF elements and incorporating elements from Dave Winer's syndication format. Libby also renamed RSS Rich Site Summary and outlined further development of the format.
This would be Netscape's last participation in RSS development for eight years. As RSS was being embraced by web publishers who wanted their feeds to be used on My.Netscape.Com and other early RSS portals, Netscape dropped RSS support in April 2001 during new owner AOL's restructuring of the company, also removing documentation and tools that supported the format.
The RSS-DEV Working Group, a project whose members included Guha and representatives of O'Reilly Media and Moreover, produced RSS 1.0 in December 2000, which reclaimed the name RDF Site Summary from RSS 0.9, reintroduced support for RDF and added XML namespaces support, adopting elements from standard metadata vocabularies such as Dublin Core. In September 2002, Winer released a major new version of the format, RSS 2.0, that redubbed its initials Really Simple Syndication.
Members of the community who felt there were significant deficiencies with this family of formats were unable to make changes directly to RSS 2.0 because the official specification document stated that it was purposely frozen to ensure its stability.
Sam Ruby set up a wiki to discuss a "well-formed log entry". People quickly started using the wiki to discuss a new syndication format to address the shortcomings of RSS in June 2003. It also became clear that the new format could form the basis of a more robust replacement for blog editing protocols such as the Blogger API and LiveJournal XML-RPC Client/Server Protocol as well. The project aimed to develop a web syndication format that was 100% vendor neutral, can be used and freely extensive by everybody, and thoroughly specified.
The discussion then moved to a newly set up mailing list. The next and final snapshot during this phase was Atom 0.3, released in December 2003. This version gained widespread adoption in syndication tools, and in particular it was added to several Google-related services, such as Blogger, Google News, and Gmail. Google's Data APIs (Beta) GData are based on Atom 1.0 and RSS 2.0. and later in the year 2004, discussions began about moving the project to a standards body such as the World Wide Web Consortium or the Internet Engineering Task Force (IETF). The group eventually chose the IETF and the Atompub working group was formally set up in June 2004, finally giving the project a charter and process.
Specifications
The RSS 2.0 specification is copyrighted by Harvard University since July 2003 and is frozen. No significant changes can be made (although the specification is under a Creative Commons licence) and it is intended that future work be done under a different name; Atom is one example of such work.
Atom 1.0 specification is structured in such a way that the IETF could conceivably issue further versions or revisions of this specification without breaking existing deployments, although there is no commitment, nor currently expressed interest in doing so.
RSS 2.0 categories have two parts: label and domain. While Atom has an additional human-readable title category.
RSS 2.0 does not specify the handling of relative URI references and is not in an XML namespace but may contain elements from other XML namespaces. There is no central place where one can find out about many popular extensions. Atom 1.0 is in an XML namespace and may contain elements or attributes from other XML namespaces. There are specific guidelines on how to interpret extension elements. Additionally, there will be an IANA managed directory values. Finally, Atom 1.0 provides recommended extension points and guidance on how to interpret simple extensions.