WC = 35.74 + 0.6215T − 35.75V0.16 + 0.4275T V0.16 (defined at or below 50°F and at least 3 mph). Heat index is the NWS Rothfusz regression in °F and relative humidity. Dew point uses Magnus: α = 17.27T/(237.7+T) + ln(RH/100), Td = 237.7α/(17.27−α) with T in °C.
Wind chill estimates how much colder moving air makes cold temperatures feel, calculated at or below 50°F with wind of at least 3 mph. Heat index estimates how much hotter humidity makes warm temperatures feel. They're opposite-direction "feels like" figures, and only one applies at a time depending on conditions.
The US National Weather Service formula used here is only valid at or below 50°F and at least 3 mph of wind. Outside that range the formula's underlying assumptions break down, so it wouldn't produce a meaningful "feels like" figure.
Dew point is the temperature air would need to cool to for moisture to condense. Unlike relative humidity, which changes with temperature even when the actual moisture content doesn't, dew point directly reflects how much moisture is in the air — which is why meteorologists often prefer it for describing how muggy it feels.
They use the same published NWS formulas — the Rothfusz regression for heat index, the standard wind chill formula, and the Magnus approximation for dew point — but this is an independent calculator, not an official forecast or warning source.