Actually, both play a role in the human comfort level.
Dewpoint is related to the quantity of moisture in the air. Dewpoints above 65 F make it feel sticky and humid outside while dewpoints less than 65 F are comfortable with respect to the stickiness of the air. The higher the dewpoint is, the more moisture that is in the air. The higher the dewpoint is above 65 F, the stickier it will feel outside (feels like you have to breathe in a bunch of moisture with each breath). 75 F or above dewpoint, the air really feels sticky and humid.
Relative humidity is the closeness the air is to saturation. When the relative humidity is less than 40%, it feels dry outside, and when the RH is greater than 80% it feels moist outside (dewpoint will determine if it is uncomfortably moist or just regularly moist). Between 40 and 80% RH is comfortable if the temperature is also comfortable.
The worst combination for human comfort is a high dewpoint (65 F or above) combined with a high relative humidity. If the dewpoint is above 65, it will generally always feel uncomfortably humid outside. The optimum combination for human comfort is a dewpoint of about 60 F and a relative humidity of between 50 and 70% (this would put the temperature at about 75 F). The air feels dry outside when BOTH the dewpoint is below 60 F AND the RH is less than 40%.
I hope that makes sense.
Dew Point exists regardless of what the humidity is. Dew Point is the temperature at which the air would become saturated (100% humidity). It can be calculated from the difference between what are called the wet and dry temperature readings at any location.
This is a good question. I am going to say the dew point. High humidity does make it feel sticky, but the dew points have to be high as well. 100% humidity in the winter won't make you feel sticky. Once the dew point gets above 70, that is when the stickyness begins to occur, no matter what the humdity is. If the temperature is 70 degrees and the dew point is 70 degrees, that would make the humidity 100%, but if the temperature is 100 degrees, the humidity would be a lot lower.
Dew point is just another way of measuring humidity. Dew point is "normalized" for temperature. So a 53 degree dew point is always equal to a 53 degree dew point. Humidity is expressed as a percent of how much the air can hold at that temperature. So a 53% at one temperature could be a 45% at another so to compare percent humidity the temperatures must be the same.
Humidity is what makes people feel sticky. In the hot summer months, if the humidity levels are high, there is more moisture in the air, which makes it feel hotter and makes people feel sweatier.
Humidity is the culprit... sorry to break it to you... it makes people feel sticky! :-P