A browser is a program that downloads HTML documents, images, and other content off the InterNet, and renders it all nicely for the user to look at. And feel free to come up with a better definition.
Examples include Mozilla (see MozillaNotes), InternetExplorer, Konqueror, Opera, MozillaFirefox, lynx... the list is endless.
Opera has the least broken implementation of CSS (which doesn't necessarily say much) and Konqueror has the most broken one. InternetExplorer just tends to be... idiosyncratic in its interpretation of the standard. (Sigh.)
The first graphical WebBrowser (from ca. 1992?) was NCSA Mosaic.
Under X on Linux I recommend Mozilla, on a Linux console I recommend lynx(1) or links(1), depending on your tastes (and resolution). --PerryLorier
For the console, I'd actually seriously consider w3m(1)? - it basically renders the page as a graphical browser would, within the constraints of a terminal. Also, nowadays, MozillaFirefox would be a serious option for browsing under X. It's also going to entirely replace "!SeaMonkey?" (the Mozilla browser's project name), of which 1.7 is slated to be the last release. --AristotlePagaltzis