Putting my flight instructor hat on here, this is mostly correct.

“Indicated airspeed” (IAS) is what number your aircraft’s airspeed indicator is pointing to. As Rivalarrival described, the airspeed indicator is a barometric instrument that compares ram air pressure with static pressure to measure the dynamic pressure, which is a function of airspeed. Indicated airspeed is an indication of how the airplane will “feel,” how much force will act upon the aircraft in maneuvers, which is why force limit speeds such as maximum flap and landing gear extended speeds, stall speeds, max normal operating speed and never exceed speed are marked on the airspeed indicator.

“Calibrated airspeed” (CAS) is indicated airspeed corrected for instrument error. The airspeed indicator and the plumbing it is hooked to aren’t perfect, so they’ll be off by a few knots especially near the lower edge of its range. You find a chart in the POH that says “IAS 45, 50, 55, 60 etc” on one line and “CAS 43, 49, 54, 60 etc” on another. Pilots use this for, if we’re being honest with ourselves, nothing.

“True airspeed” (TAS) is indicated airspeed corrected for air density. The airspeed indicator is flawed in concept: It’s a pressure gauge calibrated in units of speed. To actually determine the relative velocity of the aircraft through the air, we have to do a bit of math comparing the outside air temperature with our pressure altitude, this will give us our density altitude. You then do a bit more math to correct calibrated airspeed for density altitude and get true airspeed. E6B flight computers have little windows for this.

Here is my old cardboard E6B from when I was a student. I’ve set an air temperature of -40C over 30,000 feet in the right-hand window, the center window is showing…pretty much exactly 30k feet of density altitude, and we can read true airspeed over calibrated airspeed on the A and B scales. So for 100 knots, we can look at the 10 on the B scale, and read about 164, maybe 165 knots on the A scale. At 30,000 feet and +50C, which literally never happens, your density altitude is ~38,000 feet and 100 KCAS will get you 194 KTAS. Not quite 400 😜

“Ground speed” is true airspeed corrected for wind. To calculate your ground speed, you need your true airspeed as we just calculated, and winds aloft forecasts from one of the government agencies the Republicans are desperate to destroy, and then we do some trigonometry. You can whip out your Ti-83 Plus Silver Edition from high school and SohCahToa this bitch, or you can flip the E6B over to find a handy dandy vector plotter, which does ground speed and wind correction angle calculations by accurately drawing and measuring the triangle. My high school physics teacher called using this thing “cheating,” I call it “a required aeronautical skill.”