The Doppler Effect is the change in frequency or wavelength of a wave as observed by an observer moving relative to the source of the wave. It occurs when there is relative motion between the source and the observer.
The distance between two consecutive points on a wave.
Redshift/Blueshift: Redshift refers to an increase in wavelength (and decrease in frequency) due to an object moving away, while blueshift refers to a decrease in wavelength (and increase in frequency) due to an object moving closer.