From what I have found, NEMA is an organization which stands for "National Electrical Manufactures Association.
The 23 indicates the size of the stepper motor. There are many different sizes such as NEMA 17, 23, and 34. The larger the number the larger the motor.
Here is a diagram of a typical NEMA 23 motor:
Stepper motors come in a variety of sizes, with different types numbers of poles (wires) and of course different ratings (voltage, torque etc).
The motors I am planning on using are a set of three NEMA 23 6V motors with 6 wires. The NEMA 23 motors have 1/4" shafts which make it easy to connect to the 1/4-20 ALL-Thread lead screws.