Absolute humidity is the exact mass of water vapor in the air. As pressure and temperature change, the volume of air changes, and changes absolute humidity making it difficult to judge.
Specific humidity measures the mass of water vapor in a very specific space of air. Because it focuses directly on one air packet, specific humidity does not change like absolute humidity and it is not especially useful for measuring overall humidity levels.
Unlike absolute and specific humidity which measure an exact mass of water vapor in the air, relative humidity is a ratio. This ratio is the amount of water vapor in the air versus the maximum amount of water vapor that the air can hold. Relative humidity is a useful measurement because it can measure humidity on a large scale and still adjust easily for changes in temperature and pressure.
At a given temperature there is only so much water vapor that can be in the air. When the humidity reaches this level the air is considered saturated. When the temperature rises, the saturation level increases, and as it lowers, saturation decreases. For an equal level of relative humidity the amount of water vapor actually in the air increases with an increase in temperature.