Air conditioners and dehumidifiers are the same machine -- just different exhaust mechanisms. They both remove water from the air, thus reducing actual and relative humidity. Heaters raise the air temperature, thus reducing the relative humidity (but not, in general, the actual humidity).
Edit to clarify: "relative humidity" is the amount of water in the air as a percentage of the maximum possible capacity. "humidity," or "absolute humidity," is the molar (or other absolute unit) content of water per unit volume of air. The dew point is the temperature at which a given absolute humidity will start to condense out of the air -- thus an indication of the temperature at which the relative humidity equals or exceeds 100%. The dew point is the same everywhere for a given humidity.