"Factors" are numbers we can multiply together to get another number.
When we compare lists of factors of two or more numbers, any factors that are the same are the "common factors".
Example: 12 and 16
• The factors of 12 are: 1, 2, 3, 4, 6 and 12
• The factors of 16 are: 1, 2, 4, 8 and 16
So the common factors of 12 and 16 are: 1, 2 and 4
See: Factor