I've written an "Understanding Lucene" refcard that has just been published at DZone.  See here for details:

   http://www.lucidimagination.com/blog/2011/03/28/understanding-lucene-by-erik-hatcher-free-dzone-refcard-now-available/

If you're new to Lucene or Solr, this refcard will be a nice grounding in the fundamental concepts.  For you old timers, pass it on to your friends and coworkers :)

Erik