This is a demonstration of a waveform (top), spectrum (middle) and spectrogram (bottom) of a simple signal. Click start to begin the playback and slowly adjust the frequency. If you 'switch to Harmonic Complex' you will see a complex signal with the fundamental frequency defined by the slider and the first 5 harmonics of the signal present. If you switch back and forth between the two signal types you will see that the first component is the same for both.
note, usually the spectrogram has the axes the other way round to show time on the x-axis and frequency on the y-axis, but this is flipped for easier comparison with the spectrum.