JDT Core 

JDT Core (org.eclipse.jdt.core) is the plug-in that defines the core Java elements and API. You should always list this plug-in as a prerequisite when you are developing Java specific features.

JDT Core packages give you access to the Java model objects and headless Java IDE infrastructure.  The JDT Core packages include:

org.eclipse.jdt.core	Classes that describe the Java model.
org.eclipse.jdt.core.compiler	API for the compiler infrastructure.
org.eclipse.jdt.core.dom	Abstract Syntax Trees (AST) for examining the structure of a compilation unit down to the statement level.
org.eclipse.jdt.core.eval	Evaluate of code snippets in a scrapbook or inside the debugger.
org.eclipse.jdt.core.jdom	Java Document Object Model (DOM) that can be used for walking the structure of a Java compilation unit.
org.eclipse.jdt.core.search	Search the workspace's Java model for Java elements match a particular description.
org.eclipse.jdt.core.util	Utility classes for manipulating .class files and Java model elements.

Manipulation of the structure of a compilation unit should be done using org.eclipse.jdt.core.dom instead of org.eclipse.jdt.core.jdom. org.eclipse.jdt.core.jdom will be deprecated in the 2.2. stream and replaced with org.eclipse.jdt.core.dom.