RSS documents (generally called "RSS feeds", "news feeds" or just "feeds") can be read with an RSS reader like BottomFeeder or Feedly. These are sometimes called "aggregators" because they combine multiple RSS feeds which the user can browse as a single list. The RSS reader tracks which articles the use has read, and is typically set to show only new articles, hence the idea of a "feed" or flow of new items.
Most RSS feeds are based on RDF. RDF is a structured document format for describing textual resources such as news articles available on the web. RSS originally stood for "RDF Site Summary" as it was designed to provide short descriptions of (changes to) a website.
Because it provides a standard way to deliver, or "syndicate", news or updates from one site to another, RSS is sometimes expanded as "Really Simple Syndication". It is closely associated with blogs, most of which provide an RSS feed of articles.