Smurfjet,
4 types of airspeed:
INDICATED- what the A/s indicator says.
CALIBRATED- indicated corrected for installation error. (Airflow disturbed by a wing strut, probe, etc, can cause minor errors depending on where the pitot tube is located. POH will normally have a chart/graph showing expected error at different speeds)
TRUE- calibrated corrected for non-standard pressure/temperature.(As altitude or temperature increases, air density decreases. For any given IAS, TAS will increase with altitude)
EQUIVALENT-calibrated corrected for compression/temperature errors.
EAS only needs to be considered at high speeds, around 190 kts+.
At those speeds, the airspeed probe compresses the air in front of, and in, the probe itself (pitot tube!) This 'compacting' changes the air density and temperature, and must be corrected to yield an accurate airspeed. (Simplified version!)