Flash is a ClosedSource vector graphics and animation format, player and editor, created by Macromedia (who were acquired by Adobe). The player is by far the most common WebBrowser plugin, and a lot of Flash content floats around the web. (The majority of it is probably animated banner ads these days.) There are many WebComics done in MacromediaFlash.
The SWF FileFormat used by Flash is not open, but mostly documented. It is quite easy to create SWF content using free tools, such as Ming, SWFTools, SSWF and MTASC. The Open Source Flash website is a useful resource if you are so inclined.
You need to install the following Packages:
Edit the file /usr/X11R6/lib/X11/fs/config and make sure you have /var/lib/defoma/x-ttcidfont-conf.d/dirs/TrueType/ path listed in the catalogue line. Restart xfs (sudo /etc/init.d/xfs restart) and then your WebBrowser.
(NB.: the reference to X11R6 rather than xfree is deliberate as this is where MacromediaFlash looks.)
This information was originally found at http://www.linuxforums.org/forum/debian-linux-help/46720-flash-player-problem-2.html and then modified.