Here is a good reference on stepper motor terminology:-

http://www.anaheimautomation.com/stepper_motor_glossary.aspx

Basic explanation of stepper motors:-

http://www.solarbotics.net/library/pieces/parts_mech_steppers.html

although the final chart has a bug for port 2a. The final blue bar should be moved one place to the right !! Man – debugging other peoples code……

Also:-

http://library.solarbotics.net/pdflib/pdf/motorbas.pdf

but they also have a bug on page 4, Table 1, the bottom 3 dots in the Half-step drive should all be one column to the left.