IAS-what your instrument reads (dynamic pressure based on Sea level)
TAS--speed through the air mass corrected for Temperature and pressure (atmospheric pressure). Since the higher you go, the lower the temp and atmospheric pressure....the HIGHER your TAS. TAS and IAS should be equal at sealevel.
GS (ground speed)--speed at which you travel over the ground.
Technically, lower temperature will result in a lower TAS for the same IAS at a given pressure altitude (or, more accurately from a physics point of view, a higher IAS for a given TAS) since a lower temperature will result in denser air.
TAS is the "actual" speed through the air, but can't be measured directly by pressure instruments, and has no impact on lift or drag, as long as you are below the transonic region. Notably, one area where TAS does have an effect on aircraft performance is in thrust. As your TAS increases, thrust from a propeller will drop off for any given engine power output, if all other conditions are the same.
Mike