Wordle is a really fun (and super easy to use) visualization tool that allows a person to create a "word cloud".  It is far from new and many educators have been using it for years.  The best way to understand it is with an example... below is a "wordle" of the technology I use on a regular basis:

There are many, many ways to incorporate Wordle into your classroom and teaching.  The presentation below has a few examples and the YouTube videos below that provide examples and a bunch of tips that you might find helpful when creating your own Wordles.  One final tidbit.  You can't save a Wordle, instead you have to take a screenshot of it and then place it into a Word document, PowerPoint or whatever.  Here is a simple tutorial for how to screenshot something.  If you want to more efficient way to create screenshots I recommend SnagIt (costs $) or Greenshot (free). Have fun and enjoy.
Wordle IntroWordle - Part 1... the basics.  I did make a part 2 if you are interested.  Find that here.