The spectrogram is a two-dimensional view of the evolution of the signal’s spectrum over time, i.e. a frequency (Y-axis) versus time (X-axis) plot (or the invert; depending on the direction setting), with the magnitude modulating the color and intensity of the pixels.
A spectrogram can be computed using the STFT (short-term Fourier transform) as well as other means. It serves as a useful tool to get a global picture of how the frequency content of a signal changes over a time, and eases identification of its structure. Broadband noise appears as background, a pure tone tone as a horizontal line, and a transient as a vertical line.
Harmonic content appears as horizontal groups of parallel lines and vertical bars respectively, etc.
Example spectrogram view