Like Gothamist said yesterday, dew point is an absolute measure of how much moisture is in the air. To take one step further into weather weenie-ism, the dew point is a proxy for the portion of air pressure that is exerted by water vapor. The rate of evaporation depends solely on temperature, so you can think of temperature as measuring the maximum amount of pressure that can be exerted by water vapor.
Since air pressure is the weight of the air above the earth's surface it is impractical to directly measure vapor pressure. You'd have to grab a column of air several miles high and squeeze out all the water. Luckily, there are a couple of ways to indirectly measure water vapor, one of which uses the coolest instrument in meteorology.
One way to measure humidity is to take two thermometers, wrap the end of one thermometer with a wet cotton wick. Twirl the thermometers around in the air for a few minutes. The thermometer with the wet wick, called the, ta-dum, wet-bulb thermometer, will cool off from evaporation much as we cool off when getting out of the shower or the pool. Then, using a weather calculator you can find the dew point and relative humidity.
The other way to measure humidity is to use a hair hygrometer. Hair is made of keratin, a protein that is wound up like a coil. The chemical bonds that hold the coils together break in the presence of water. The coil loosens and hair gets longer with increasing humidity. The relationship between how much hair lengthens and humidity is well known, and a hair hygrometer is little more than luxuriant flowing hair stretched between a stationary point and a moving pointer. Heck, Gothamist encourages everyone to make their own and report their humidity readings back to us.