What is 'Doppler effect' ? Give illustrative examples?

The phenomena of apparent change in frequency of sound heard by the observer due to relative motion between the source and observer is called Doppler effect.
Ex: 1) When a whistling train is approaching a station, for a peroson standing on the platform, the frequency of sound appear to increase because the train is in relative motion with respect to observer
${\mathrm{n}}^1=\mathrm{n}\left[\frac{\mathrm{v}}{\mathrm{v}-{\mathrm{v}}_{\mathrm{s}}}\right]$
2) When the whistling train is receding away a station the frequency of whistle appear to decrease for the stationary observer
${\mathrm{n}}^1=\mathrm{n}\left[\frac{\mathrm{v}}{\mathrm{v}+{\mathrm{v}}_{\mathrm{s}}}\right]$