There are many explanations of RSS and OPML at the conceptual level, for example, here and here. Not too many people want to digg their physical realization, which is XML, though XML is human readable. Conceptual model is more important for people to further leverage these two standards.
We’ll try to explain them here using metaphor. Hope it’ll help understanding them easier.
1. Comparing with TV.
RSS is summery of individual channels. And OPML is the channel (TV Guide Channel) for other channels. Every RSS contains one CHANNEL and may have many ITEMS. From TV perspective, ITEMS are PROGRAMS. On the cable TV at my home, the channel for other channels is 22, which outlines program information about what is going on in other channels.
2. Comparing with a research paper.
RSS item is abstraction part of a research paper. OPML is reference part of a research paper. For example,
“Understanding Data Through Analysis of Structured Markup and Usage
Steven J. Altschuler; Edward Jung; Lani F. Wu
Data and documents with semantic markup (such as in XML) allows for the construction of usage models that are human interpretable. A framework for performing analysis over semantic markup data in conjunction with usage information is described along with applications for performing automated clustering and “find-similar”. A strong emphasis on “human interpretability” is placed on the results of these analytical techniques.
Keywords: XML, interaction history, semantic analysis, clustering, pattern lattice”
It contains title, authors, publish date and abstraction is the short description.
S. Chu, G. Ellis, Investigating Lattice Techniques for Organising Document Collections, In the 4th International Conference on Conceptual Structures, ICCS’96, 1996 (RMIT University, Australia).Paul S. Jacobs, ed., “Text-based Intelligent Systems”, Lawrence Erlbaum Assoc., New Jersey, 1992Yezdezard Zerxes Lashkari, Feature guided automated collaborative filtering, Thesis (M.S.), MIT, Program in Media Arts & Sciences, 1995.
J. H. Van Lint, R. M. Wilson, “A course in combinatorics”, Cambridge University, 1992.
This reference (or bibilography) part outlines other research papers, similar to OPML.
3. Comparing with web site.
RSS is summery of an individual web pages. OPML is the sitemap. From this perspective, “Rich Site Summary” is a better name than “Really Simple Syndication”.