RSS is a three letter acronym for either:
RSS is an XML format for syndicating content. It provides facilities for listing channels of syndicated information and items within that channel. The items may describe meta-data, content data or both.
RSS is a family of XML specifications for encoding syndicated information flows. The two versions of RSS currently in circulation in 2005 are RSS 1.0 and RSS 2.0. They are not directly related to each other, though they descend from a common root. Nor are they compatible (they use, for example, different root elements for the XML document). Efforts are underway, however, to help both formats to coexist reasonably peacefully. RSS 1.0 is an RDF (Resource Description Framework) application. RSS 2.0 is a pure XML application. Hence the acronym is expanded to Rich Site Summary, or RDF Site Summary among other variations. Both RSS 1.0 and 2.0 are primarily designed to be lightweight publication formats. The choice of RDF properties or XML elements is chosen to represent that goal. RSS 1.0, however, has an explicit concept of modules, that plug-in to the standard to provide for more representation that is part of the core. For example, there is no provenance meta-data in RSS 1.0, but there is a Dublin Core module defined as a standard extension.
Conceptually, RSS defines channels of information, along which individual items flow. An item may contain a link to another web resource, plus various metadata and optionally content. There are no formal relationships between channels. However, it is common for blogs to be partitioned into categories, where an item might be part of multiple categories and form part of that site's overall flow.