factors of a number are those numbers that can divide the number exactly without leaving a reminder.
using 6 and 12 as an example
factors of 12 {1,2,3,4,6,12}
factors of 6 { 1,2,3,6}
then the common factor from both 6 and 12 are { 1,2,3,4,6}
and from this , the greatest common factor is 6.