Both the effects (lengthening of the transmitted signal's wavelength and decrease in its frequency) indicates that the object is moving away from the observer, and this phenomenon is called Doppler effect.
In fact, when the object is moving away from the observer, the relative distance between two consecutive crests of the wave emitted by the object increases to the observer eyes, since he's moving away. This means that the wavelength appears larger, and since the frequency is inversely proportional to the wavelength, the frequency appears smaller.
