2 Answers

FACTORS  are the numbers that you multiply to get a PRODUCT.  The PRODUCT is the answer in multiplication.  A SUM is the answer in addition, remainder in subtraction, and quotient in division.   
to find the greatest common factor, you do what chelleanne said..... list all the numbers that can be multiplied to get to that product
12 x 1,   6 x 2,  4 x 3           1, 2, 3, 4, 6, 12  are factors of 12
18 x 1,  9 x 2,  6 x 3             1, 2, 3, 6, 9, 12 are factors of 18
The largest number that is a factor of both 12 and 18  is 6, which means 6 is the largest common factor.   (Note:  1 will always be the lowest common factor of any numbers) 

Find the GCF(greatest common factor) of 12 & 18.

12 = ( 1, 2, 3, 4, 6, 12 )

18 = ( 1, 2, 3, 6, 9, 18 )

The greatest number 12 & 18 share is 6.

6 divides evenly into 12 & 18