A is for Assimilation. Achieved through means of nanoprobes.
B is for Beard. Presumably, no Borg has one. (Anything to avoid saying the obvious 'Borg')
C is for Cube, their ships are.
D is for Delta Quadrant, the primary residence of the collective.
E is for Eye, you can count on one of them being replaced or enhanced.
F is for FULL REVERSE! Often yelled by the XO of a starship once the Borg has it within a tractor beam. (note: it never works)
G is for Green. The color of Borg cutting beams.