Temperature alone lies. Wind takes heat off you far faster than the number suggests, humidity stops you cooling yourself down, and wet clothing changes everything again. This works out what the air actually feels like, how long you have before frostbite or heatstroke becomes likely, and what to do about it — using the formulas weather services and mountain rescue actually use.
Wind chill uses the North American/European standard
13.12 + 0.6215T − 11.37V0.16 + 0.3965TV0.16 with T in °C and V in
km/h, measured 10 m up and on a face. It is only defined below 10°C and above about 5 km/h; outside that
range this tool shows the air temperature instead of inventing a number. It says nothing about frostbite on
covered skin, or about a wind you are sheltered from.
Heat index is the Rothfusz regression with the standard low-humidity and high-humidity corrections, computed in Fahrenheit then converted. It assumes shade: in full sun the real load is roughly 8°C higher, and it assumes you can still sweat.
Wetness is applied as an effective-temperature penalty rather than a formula, because the honest answer is that water conducts heat roughly 25 times better than air and there is no simple correction that covers everything from damp socks to full immersion. Treat the wet column as a warning, not a measurement. The single most useful line on this page is the one about getting wet clothes off.