The Atom syndication format is similar to Rich Site Summary (RSS). RSS and Atom are both formats designed to syndicate headlines or articles from websites or information systems, in a kind of format where you see (headline) (date) (author) (content). These are interpreted by either plugins to blogging systems, RSS aggregators, or RSS readers. They allow a kind of thin client model to browsing lots of sources of information quickly, without having to go through all of the stylistic decisions and heavier content of the main website. Atom is oriented more towards weblog syndication and its model of categories, authors, and integrated metadata, whereas RSS is more oriented towards web site headlines. You can think of Atom as the progeny of RSS, although Dave Winer, the author of RSS, would not agree. Atom was primarily engineered in order to provide more of a standards-based development process to the syndication format arena, providing a more open model than Dave Winer's single stewardship of RSS. They both are fundamentally similar in functionality.
Technically, Atom is attempting to bring some rigidity to the "everything's optional" world of RSS. It adds some structure and definitions to the content allowed in specific elements, whereas RSS aims to be as "undefined" as a specification can be, allowing more flexibility for the developer but less for the user in interpreting the data. It also simplifies the technical aspects of accessing an RSS feed, essentially saying HTTP is the protocol instead of the hodgepodge that makes up RSS (HTTP, XML-RPC, all the different RSS versions, the different blog APIs, etc).
Atom intends to provide specifications for, among other things,
- Encoding content (how the data is interpreted by a program)
- Internationalization (I18N, Unicode) - currently RSS is technically focused around ASCII only
- Clear "plugins" and extensions, offering more features to the Atom format. Rather than RSS's attitude of "allow everything", Atom says, "Allow the base model, and if the user has these plugins loaded, do x with the data.
- Simplifying the data model (same APIs for both syndication and blog updating), making Atom more focused around the data itself rather than XML-RPC/RSS's model of focusing around the delivery.
Atom's development is led by Sam Ruby and is/was also called by the names Echo and Pie.
Examples of RSS (as well as Atom) readers and systems include: