title: What is the Groovy API, and what are these classes used for?
body: <p>I'm not asking about the "Groovy JDK API Documentation", which consists of about 100 classes, most of them pre-existing Java classes, and showing the enhancements of them which Groovy provides. </p>
  
  <p>When you download "documentation" from the <a href="http://groovy-lang.org/download.html" rel="nofollow noreferrer">Groovy download page</a> and unpack you find 4 folders under "html". One is called "groovy-jdk" and is the above Groovy JDK API Documentation.</p>
  
  <p>But another is called "gapi" (frames) and another "api" (no frames) and when you click on index.html of either a Javadoc shows hundreds of baffling classes: it starts in the frame version with <code>classorg.codehaus.groovy.syntax.ASTHelper</code>, <em>"A common base class of AST helper methods which can be shared across the classic and new parsers"</em>, and goes on to (Script) <code>org.codehaus.groovy.classgen.genMathModification</code>. Online version (without frames) <a href="http://docs.groovy-lang.org/docs/latest/html/api/" rel="nofollow noreferrer">here</a>.</p>
  
  <p>Can anyone explain what these hundreds of classes are, who uses them and under what circumstances? Are they just new classes introduced by Groovy but which don't happen to be enhancements of existing Java classes?</p>