With the exception of continuous wave Doppler, all diagnostic ultrasound modalities use pulsed ultrasound. By using pulsed ultrasound, the computer can time each pulse-echo sequence and thereby accurately determine the location of the interface that produced the echo. To understand pulsed ultrasound you need to have a good understanding of the following five concepts:
pulse duration
spatial pulse length
pulse repetition period
pulse repetition frequency
duty factor
To help you with this understanding, I have created the following four videos: