An RSS 'feed' is really just a document that lives on the web; it's
updated to contain that latest stuff the site has. RSS readers
download that document and present the info to the user. It's much
smaller than most web pages, since it only contains the site summary.
From the BBC World News feed, here's a snippet of such a document:
<item>
<title>Andaman coral 'hit by tsunami'</title>
<description>India plans to initiate an assessment of the damage caused
by the tsunami to the pristine coral reefs of the Andaman and Nicobar
islands.</description>
<link>
http://news.bbc.co.uk/go/click/rss/0.91/public/-/2/hi/science/nature/4148235.stm</link>
</item>
Some feeds may have more information in them than that, but that
gives the idea. It's much quicker to scan the headlines in an RSS
reader than to dig around at the BBC site looking for news (and
viewing images in the process). If there's a headline of interest,
a click downloads the full story.