A thermistor is a small bead of metal oxide whose resistance changes precisely with temperature. NTC ("negative temperature coefficient") thermistors — the common kind — drop in resistance as they heat up. A typical 10 kΩ NTC reads 10 kΩ at 25 °C, ~32 kΩ at 0 °C, ~3 kΩ at 60 °C.
From ADC count to degrees
The wiring is the familiar voltage divider. The maths is one step further: convert the ADC count back to a resistance, then plug into the Steinhart-Hart equation. The equation looks scary but it's just a curve fit — three constants A, B, C published in the thermistor's datasheet, then a logarithm and some arithmetic.
The simplified Beta equation is good enough for most hobby uses: T = 1 / (1/T0 + ln(R/R0)/B) - 273.15, where T0 = 298.15 K (25 °C in kelvin), R0 = the thermistor's rated resistance at 25 °C, and B is its "beta coefficient" (typically 3500–4000). Fifteen lines of code; results within a degree.
Why "thermistor" is the right answer surprisingly often
You'll see dedicated I²C temperature chips (TMP102, BMP280) and clever digital one-wire sensors (DS18B20). They're nice. But a thermistor costs ten cents, has zero protocol overhead, lives in any analog pin, and survives temperatures and chemistries that would kill an IC. The 3D printer keeping the extruder at 200 °C and the kettle that turns off when boiling: both thermistors.