See below. Ekg: shows the electrical activity of the heart and the heart rhythm echocardiogram: an ultrasound, shows the actual structures such as the heart muscle and the valves and with a doppler the direction and velocity of blood flow.
Electro vs. Echo. Like most medical terms, these terms are simply combinations of label words. Cardio refers to heart. Electro refers to electricity (as a result of differential heart muscle activity, as recorded at skin surface). Echo refers to reflections of sound waves, typically 1-10mhz, sent through the heart, valves & blood; all variants of water density. These help see limited aspects of our bodies behaviors.