PW stands for pulsed wave Doppler and CW for continuous wave Doppler. They are both forms of spectral Doppler and have important differences and use
Pulsed Wave Doppler (PW)
PW allows us to measure blood velocities at a single point, or within a small window of space. It requires the ultrasound probe to send out a pulsed signal to a certain depth (chosen by the operator) and then stay quiet and just listen for the reflected frequency shift from that particular depth. The computer then calculates the velocity of flow at the chosen point. Because the machine has a waiting time to listen for a return, there is limit to how fast it can accurately measure the velocity of blood flow.
Above is an example of PW at the left ventricular outflow tract (LVOT) in the apical long axis. The machine plots velocities (y axis shown in cm/s) over time (x axis).
The baseline is shown as an orange line going across. Points below the line represent velocities going away from the transducer. Points above the orange line show velocities going toward transducer. So we can deduce from above that there is a pulsatile flow going away from the transducer in systole (right after the QRS on the EKG tracing in green).
The computer has already translated all of the Doppler frequency shifts it sees into velocities using the Doppler shift equation. Each velocity is plotted as one white point. Many white points (from many doppler shift frequencies) make up the nice flow profile shown above.
The more intense the density of white points, the stronger the signal returning to the probe at the frequency/velocity.
The close up on the right (circled in red) shows exactly where the machine is measuring velocities, in between the two small horizontal lines, as determined by the sonographer. Note the "outlined" shaped of the LVOT jet during systole in the PW image above. The narrow window of PW often creates this appearance because with laminar flow there is a small set of velocities measured at any given time point.
Continuous Wave Doppler (CW)
CW allows us to measure blood velocities along an entire line of interrogation. It requires the probe to continuously send out pulses of ultrasound along a line and continuously listen for the multitude of reflected frequency shifts that are coming back. In other words the ultrasound probe is never quiet and just listening for returned frequencies as with PW; it is both sending and receiving all of the time. Because of this, we are able to use continuous wave Doppler to pick up very high velocity flows, ones which PW cannot accurately measure. Unfortunately the disadvantage is that we cannot pinpoint where along the line that velocity is coming from. Hence a combination of PW and CW are used in many cases to pinpoint both velocity and location of a particular blood flow jet of interest
This is an example of CW across the LVOT and aortic valve in the 5 chamber view in someone with aortic stenosis. Note the high velocity jet of almost 4 m/s, which only CW is able to demonstrate. We know using PW above and below the valve (and inference from 2D images showing a stenotic aortic valve) that the blood flow acceleration is at the level of the aortic valve.
This is an example of the Piedoff probe used at the right upper sternal border. The Piedoff probe is a special probe designed only to do CW. It has two crystals, one for transmitting continuous ultrasound and one for receiving and is sometimes used in cases of aortic stenosis to get the highest velocity jet. Note there is no 2D picture to help guide the positioning of the probe, which instead is dependent on sonographer experience.