See:
Description

JET

org.eclipse.jet	Defines core interfaces for executing JET transformations and templates.
org.eclipse.jet.compiler	Defines an AST for JET templates, and provides a compiler that generates an AST from a template source.
org.eclipse.jet.runtime.model	Defines interfaces for loading models into memory for access by JET templates (incomplete).
org.eclipse.jet.taglib	Interfaces and abstract classes for implement custom JET tags.
org.eclipse.jet.taglib.java	Define utility functions for interacting with the Standard JET Java tags.
org.eclipse.jet.taglib.workspace	Defines JET workspace actions that custom tags may perform as part of a JET transformation.
org.eclipse.jet.transform	Defines JET transformation related interfaces.
org.eclipse.jet.xpath	The JET XPath processor.
org.eclipse.jet.xpath.inspector	Interfaces allowing in-memory models to be handled by the JET XPath processor.

Other Packages

org.eclipse.jet.core.compiler
org.eclipse.jet.core.expressions
org.eclipse.jet.core.parser
org.eclipse.jet.core.parser.ast
org.eclipse.jet.ui
org.eclipse.jet.ui.newproject