Officially, the dewpoint temperature defines the temperature
which a parcel of air would need to be cooled at constant pressure and constant moisture content in order for the vapor
pressure and saturation vapor pressure of the parcel to be equal. In other words, it's the temperature at which a parcel of air would be saturated (i.e.,
100% relative humidity).

In simpler terms the dewpoint temperature is what we use to get
a better idea of the humidity outside. You already know that the more water there is in the air, the more “muggy” it feels. And now you know that
dewpoint is a way of measuring how much water there is in the
air. So if you know the dewpoint, you know how muggy it will feel when you go outside.

Dewpoint is a better measure of “mugginess” than relative
humidity because the dewpoint isn’t affected by changes in the
temperature. Look at these two examples… On a day where the temperature is 40° and the dew point is 37°, the relative humidity is 90%. Even though the
relative humidity is high, it would not feel “muggy”. On a day where
the temperature is 95° and the dew point is 75°, the relative
humidity is only 50%. But this day would feel very muggy. Warm air holds
more water than cold air, so a relative humidity of 50% on a hot day
is a lot more humid than a relative humidity of 50% on a cold day. You
can think of a warm day as being a large pitcher capable of holding
a lot of water and a cold day as maybe a glass capable of holding a lot
less water.

And here’s a dewpoint scale (in degrees F) that will help you…

Less than 50....Not Muggy
50 - 59....….....slightly muggy
60 - 69....….....moderately muggy
70 - 79...…......very muggy
>79 .................. unbearable

The following web site has some good explainations about
humidity in general and can be found at http://www.unc.edu/depts/cmse/nature/dewpoint.html

http://www.erh.noaa.gov/er/buf/gfe/definedewpoint.html

Dewpoint can be measured by an instrument called a hygrometer. The simplest
hygrometer - a sling psychrometer - consists of two thermometers mounted
together with a handle attached on a chain. One thermometer is ordinary, the
other has a cloth wick over its bulb and is called a wet-bulb thermometer.

When a reading is to be taken, the wick is first dipped in water and then the
instrument is whirled around. During the whirling, the water evaporates from
the wick, cooling the wet-bulb thermometer. Then the temperatures of both
thermometers are read. If the surrounding air is dry, more moisture
evaporates from the wick, cooling the wet-bulb thermometer more so there is a
greater difference between the temperatures of the two thermometers. If the
surrounding air is holding as much moisture as possible - if the relative
humidity is 100% - there is no difference between the two temperatures. An
easy way to make your own sling psychrometer can be found at
http://www.miamisci.org/hurricane/psychrometer.html; the resulting table gives
you relative humidity and there are many calculators that can convert that to
a dewpoint temperature; a good site for such calculations can be found at
http://www.srh.noaa.gov/elp/wxcalc/wxcalc.shtml

There are also commercial sling psychrometers as well as handheld digital
hygrometers available on the market, that are a bit easier to use than a sling
psychrometer and seem somewhat affordable; suggest doing a search on Google
(http://www.google.com) using the keyword of hygrothermometer or "sling
psychrometer".