A news aggregator or news (feed) reader is a computer program which tracks syndicated information feeds, via RSS, RDF, or Atom. Most news aggregators allow one to 'subscribe' to a feed, and automatically keep track of the articles one has read, similar to an email client tracking read emails.

Many blogs make their content available in feed form for the convenience of readers using news aggregators. WordPress can generate feeds in RSS and/or Atom formats.

Reference http://codex.wordpress.org/Glossary