Subtle difference between WebServer and Application server is handling diff J2EE componenets. WebServer is only able to serve data/sources which mainly being used to generate dynamic response(s) (i.e. html, jsp, servlets) whereas Application server can also support EJB. HFJ is having nice para on this, you should go through it once, it will clear all your doubts.
Sorry for my abbreviation, I should write it in more meaningful way. Its a book suggested by JavaRanchers for Sun Certified Web Component Developer(SCWCD) exam, book's name is Head First JSP & Servlets (HFJ).