JDT stands for Java development tools, the sub-project of the Eclipse project that develops tools for programming in Java. This includes a Java compiler, incremental builder, editors, wizards, content assist, and all the other features of a first class Java IDE. For more information, see the JDT web page.
The JDT subproject is broken down into components. Each component operates like a project unto its own, with its own set of committers, bug categories and mailing lists.