could someone help me understand this? i have no idea how to answer it. the question is:

how can you tell when two places A1x +B1y +C1z=D1 and A2x+B2y+C2z=D2 are parallel? perpindicular? (the 1's and 2's are sub 1 and sub 2, i just don't know how to type that. if it makes any difference...)

if anyone could give me some help to clarify this it would be much appreciated. thanks.