1. The instrument reads Indicated Air Speed (IAS)
2. Take out the instrument error (Eg. the bent needle) and you get Rectified Air Speed (RAS). (The instrument error is normally minimal, although ASIs only have to be good to plus or minus a couple of knots.)
3. Then take out the Pressure Error Correction and you get Calibrated Air Speed (CAS). In modern large aircraft this will be what appears on your primary display and is then synonymous with IAS.
4. Then take out the scale altitude correction and you get Equivalent Air Speed (EAS) which is the speed at sea level which would feel the same to the aircraft.
5. Then apply a density correction and you get the True Air Speed (TAS) which is the actual speed through the air.
6. With TAS and the wind you get True Ground Speed (TGS) and with TAS and the temperature, which governs the speed of sound, you get Mach number.
7. The pilot these days is unlikely ever to need RAS because the instrument error will almost always be negligible or will be taken out in the ADC before he ever sees it. On some older generation aircraft the difference between IAS and CAS can be significant. CAS is the "truth" as far as ATC and other aircraft are concerned. Eg. If two aircraft are in formation they will both be at the same CAS even though their ASIs may be showing different, and possibly both erroneous IASs.