In my 1st venture into stepper-land, I too was initially confused with voltage ratings. It seems that no matter what voltage a stepper may be rated for, in almost every case they are run off a much higher supply to increase response speed and torque, but it is the current parameter that is most meaningful and the one that is controlled.
The sense resistors are simply very low value power resistors that are in series with the motor (usually at the connection right before ground) that allow measurement of the current flowing in the motor by measuring the small voltage difference that is developed between the resistor terminals. This sense voltage is usually fed back to the control device (like an L297) that uses the information to adjust it's pwm signal, thus controling the motor current.
fwiw you can get some small inexpensive steppers at places like Jameco, complete with documention.
Also, bipolar, though slightly more complex control hardware, is the preferred type because they produce torque more efficiently.