A newsfeed is an real-time source of news related to an website. A user can directly subscribe to this feed and keep up with the latest news without having to access the website frequently.
To read the newsfeed you need an appropiate viewer/reader. Some browsers have this functionality built-in (Internet Explorer 7, FireFox, ...) but in other cases you may have to use an independent newsfeed reader.
RSS vs Atom
What is known collectively as RSS newsfeeds actually are different and incompatible standards. There is RSS in strict sense (various versions) and Atom. Most newsfeed readers accept both formats. This website currently implements two formats: Atom 1.0 and RSS 2.0.