RSS and Atom are two different specifications for defining XML files which can be used to distribute syndicated content on the Web. RSS and Atom files are usually called ‘feeds’ though sometimes they may also be called ‘channels’. People can ‘subscribe’ to feeds in order to be alerted when new items are added to it.

Typically an RSS or Atom feed will be used to alert subscribers about updates to a web site or Blog. For example, the Bitwise feed will tell you when a new article is added to the Bitwise site; most Blog feeds tell you when a new Blog entry has been added. In both cases, when you double-click an item in your feed reader, you will be taken to the matching page in the web site or Blog.

There is, however, no requirement for the items in a feed to be associated with web pages. It is perfectly possible to create a ‘standalone’ feed which has no links to the web. A feed of this sort could be used to create a syndicated newsletter. However, in most feeds most of the time an item is linked with a web page.

What Is RSS Used For?

The explosion in weblogs (‘blogs’) has largely been responsible for launching RSS as a mainstream technology on the Web. Due to the incremental nature of weblog writing (some authors add new entries infrequently whereas others add several entries daily) it is often inconvenient to have to keep browsing back to a blog on the off chance that something new has been added. It is much more convenient to be notified when new items are added and decide whether you want to read them.

RSS is ideal for relaying regularly updated news of any type and it is now widely used by commercial news organisations such as the BBC, CNN and newspapers such as The New York Times and The London Times.

In recent times, RSS has started to be used to syndicate multimedia - particularly audio files for ‘podcasting’. RSS 2.0 is the format used for distributing Apples iTunes.

Versions of RSS

When people talk about RSS for content syndication, generally they are referring to RSS 2.0. However, there is also a different form of RSS called RSS 1.0. This is not, as you might expect, the ‘previous version’ of RSS 2.0 but is, on the contrary, a different version of RSS which is still alive and kicking. This situation arose when two strands of RSS development emerged from earlier versions of RSS (0.90 and 0.91). While RSS 2.0 is the most commonly used format for content syndication, supporters of RSS 1.0 argue that it offers additional features, such as extended support for ‘metadata’ terms which can be used to describe an article; these are useful for some types of syndication such as scientific feeds containing citations.

At heart, both RSS 1.0 and RSS 2.0 are pretty much the same kind of thing. They describe XML files which are formatted using a defined set of tags. XML is a markup language, similar to HTML but more rigorous. It contains elements between pairs of tags such as <title> and </title>. The various formats differ in the names and variety of the tags and the ways in which they are interpreted when encountered by a reader application. So, for example, when you add an item to either format of RSS feed it is enclosed between <item> and </item> whereas in an Atom feed, it is enclosed between <entry> and </entry>.

This is what the raw XML code of an RSS 2.0 Feed looks like. Here the Bitwise feed is displayed in Internet Explorer 6. Notice the paired tags such as <item></item>. These delimit the various elements of a feed so that they can be handled by reader applications.

Here is the same feed as displayed by a reader application (in this case, Awasu). The titles of each item are displayed as a list while the web page that is linked to the current item (as defined by the <link> element in the XML code) is displayed in the bottom pane.

OK, so which of the three formats is best? There is no simple answer to that question. They each have their strengths and they each have their advocates and critics. In broad terms, you might say that RSS 2.0 is the simplest format, whereas RSS 1.0 is the most complete. Atom is the newest and, some say, potentially the neatest format, but it is still early days and is not yet as widely used as RSS 2.0.

If you just want to add an RSS feed to your web site, RSS 2.0 is probably the safest (most widely supported) format to use unless you know of good reasons for adopting one of the other formats. On the other hand, if you have a Blog at www.blogger.com, you will find that your feed will be automatically provided in the Atom format.

The Jargon

Aggregator - a program that gathers and displays feeds - either to display on a web site or in a standalone ‘reader’

Atom - an XML format used to define syndicated content. A newer alternative to RSS.

Channel - another name for a feed

Content Syndication - sharing information (often the contents of a web page or Blog) with other applications or web pages

Enclosure - the <enclosure> tag in RSS is an optional sub-element of the <item> tag and it is used to describe media content such as video or audio

Feed - a specially formatted file which allows the distributing of information or ‘content syndication’

Metadata - information about information. In a feed this may include data such as the author’s name and copyright

OPML - Outline Processor Markup Language - yet another XML format, it can define hierarchical outlines of data and it is sometimes used for syndicating lists of related RSS feeds

Podcasting - distribution of audio (usually MP3) files by syndicated feeds - the term is short for “iPod broadcasting”

Reader - a program that accesses and displays RSS or Atom feeds

RSS - Really Simple Syndication or Rich Site Summary - an XML format used to define syndicated content.

XML - Extensible Markup Language - a format used to create human and machine readable hierarchical documents. Elements in XML are categorised between pairs of tags such as <title>This is My Title</title>

For more on the history of RSS see:

http://www.rss-specifications.com/history-rss.htm
http://www.webreference.com/authoring/languages/xml/rss/1/
http://en.wikipedia.org/wiki/RSS_(protocol)

For a concise reference to the RSS specification see:

http://blogs.law.harvard.edu/tech/rss

For information on Atom and the applications and feeds which support it., see:

http://www.atomenabled.org/