Signals and slots are used for communication between objects. The signals and slots mechanism is a central feature of Qt and probably the part that differs most from the features provided by other frameworks.
Signals are emitted by an object when its internal state has changed in some way that might be interesting to the object's client or owner. Only the class that defines a signal and its subclasses can emit the signal.
When a signal is emitted, the slots connected to it are usually executed immediately, just like a normal function call. When this happens, the signals and slots mechanism is totally independent of any GUI event loop. Execution of the code following the emit statement will occur once all slots have returned.
Signals are automatically generated by the moc and must not be implemented in the .cpp file. They can never have return types (i.e. use void).
The official Qt documentation can be found here for Qt 4.8 and here for Qt 5 (note about new signal syntax in Qt5). Also, here is documentation for using signals for QML.
