RSS is an easy, standardized way for people to share content.
There are several versions of RSS; RSS 0.9, RSS 0.91, RSS 1.0, RSS 1.1, and RSS 2.0. Each version tries to be backward compatible, while adding new features and functionality.
In addition to the several RSS standards, there are two other related standards, ATOM and OPML.
ATOM, like RSS is a standardized way to share content. ATOM was created in attempt to resolve some of the shortfalls of RSS.
ATOM specification, Example
OPML was created to allow multiple RSS feeds to be combined into one file. Similar to an index for the selected RSS feeds.