Human ear can hear sound of frequency between 20Hz and 20,000Hz (20KHz). This range of frequency is called the audible spectrum. Any frequency of sound above 20KHz is referred to as ultrasound. Medical diagnostic ultrasound machines usually use frequencies in the range of 1,000KHz to 20,000KHz (20MHz). The transducer of the ultrasound machine emits these high frequency ultrasound waves. These ultrasound waves then penetrate the body to a variable extent. Many of these waves get reflected. The ultrasound machine analyzes these reflected waves to create the image of the internal parts of the body. Diagnostic ultrasound is used to obtain images of the internal parts of the human and animal bodies. Diagnostic medical ultrasound of often simply referred to as ultrasound or sonogram.
Echocardiogram refers to ultrasound of the heart. "Echo" refers to ultrasound.
Doppler if based on the principle that if an object is moving, sound waves reflected from the moving object is altered in frequency. This principle is used to analyze blood flow through various parts of the body, especially the heart. The reflected altered ultrasound waves can be used to generate a graphical tracing or can be used to produce color flow images. These can be used to determine direction and velocity of the blood flow.