A way of visualising sounds that provides information about the frequencies that make up the sound.
The horizontal axis of the spectrogram displays Time (the further to the right the further along in time you are).
While the vertical axis displays the relative Frequency (the higher the image appears on the spectrogram then the higher in pitch it is).
The stronger the colour then the louder the sound.
Creating your own Spectrograms
There are many free spectrogram programmes available. One good example is the programme Raven made by researchers at Cornell University.