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.