GCF is greatest common factor, for example the GCF of 4 (1, 2, 4) and 12 (1, 2, 3, 4, 6, 12) is for because that is the biggest factor they have in common.
LCM is lowest common multiple, for example the LCM of 4 (4, 8, 12, 16, 20, 24, etc.) and 12 (12, 24, 36, 48, 60, etc.) is 12 because that is the smallest multiple they have in common.
You really need to know the difference between a factor and a multiple to understand it.
Hope this helps. :)
