What is the use of Q_OBJECT macro?

I just started using Qt and noticed that all the example class definitions have the macro Q_OBJECT as the first line. What is the purpose of this preprocessor macro?

Ans: It simply tells the pre-compiler that this class has gui elements and needs to be run through the 'moc' you only need to add this to classes that use the signal/slot mechanism.
But it will be quietly ignored in any other classes - it just adds to the build time.

Any further answers are welcome