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.
Waveform - amplitude vs time.
Spectrum - amplitude vs frequency.
Spectrgram - amplitude (colour) vs frequency (y-axis) and time (x-axis).
Note: spectrogram time is on the x-axis and frequency on the y-axis.