what is the difference between the two and when an aircraft is flying at altitude what is its actual speed

RickJ

From here:

Airspeed indicator (ASI)

a) ASI is a differential pressure guage that measures dynamic pressure, the difference between static and total, or ram, pressure caused by movement of the aircraft through the air

b) Corrugated phospor-bronze aneroid, or diaphragm, receives ram pressure from the pitot tube, while the sealed instrument case receives static pressure

c) Indicated airspeed (IAS) is the uncorrected airspeed shown on the ASI dial

d) Calibrated airspeed (CAS) is IAC corrected for instrument and position errors; POH provides CAS for IAS with various flap and landing gear configurations

e) Equivalent air speed (EAS) is CAS corrected for compression of air in the pitot tube (same as CAS in standard atmosphere at sea level; less than CAS at higher altitudes and faster airspeeds)

f) True airspeed (TAS) is CAS corrected for nonstandard temperature and pressure

i) TAS = CAS in standard atmosphere at sea level
ii) Some true airspeed indicators have temperature-compensated aneroid bellows
iii) Others have a knob requiring pilot input to align outside air temperature (OAT) with pressure altitude so that TAS is then indicated

Chperplt

In english....

Indicated airspeed is the speed that is read off of the airspeed indicator. True airspeed is indicated airspeed that is corrected for altitude (non standard pressure and temperature as we climb). As a general rule, true airspeed will increase about 2% for every 1000 feet of altitude above sea level.

Equivalent airspeed is an airspeed that has taken into account errors within the system due to the compression within the pitot tube. It's an airspeed that is never used in every day aviation.

As an aircraft climbs to the higher altitudes, the indicated airspeed decreases while the true airspeed will increase. For example, at 35,000 feet you may have an indicated airspeed of 260 knots, while the true airspeed can be 450 knots.