libsensors0 is a library to read temperature/voltage/fan sensors
lm-sensors: "Kernel drivers to read temperature/voltage/fan sensors. This is a module for reading the temperature/voltage/fan sensors in Linux via the LM78/79 chip and possibly sensors on the SMBus (System Management Bus, usually found in P6 and P-II systems). The LM80 and a LM78-clone called W83781D are also supported."
ACPI, see APM chapter
RTSensors can be configured by the user as an expert controller: The user can specify the max/min speed of the system fans, the maximum affordable temperature and so on. The controller tries to reduce fan speed to reduce noise while the temperature is in a safe range chosen by the user. So fan speed is modified automatically by the control algorithm, you don't have to use those mechanic or thermal regulators on your fans anymore.