Heterodyne bat detectors are the type that most people start off using. They are tuneable - you select the frequency range to listen to, and bat calls at that frequency are converted to sounds which you can hear. This makes them ideal detectors for immediate identification of bats in the field.
They work by filtering the ultrasonic (and therefore inaudible) bat sound with an ultrasonic signal from the detector, and producing the audible difference between the two sounds.
The bat calls are picked up by an ultrasonic microphone and mixed with the output of a high frequency oscillator in the bat detector. This produces sounds that are the sum and difference of the two frequencies. Thus if the bat detector is set to 50kHz and an incoming bat call has a peak frequency of 49kHz then the difference is 1kHz which we can hear. In theory if the bat call is at 50kHz then we hear nothing, as there would be no difference between the two signals. However, bats never emit a precisely steady sound, so there will usually be some difference between the two signals resulting in an audible sound from the detector.
Sounds can be recorded for reference but they are not suitable for sound analysis using computer software. They are the cheapest of the detectors and are simple to use, making them ideal for beginners.
Listen here for examples of calls heard on a heterodyne detector: