A common multiple is a number that is a multiple of two or more numbers. The common multiples of 3 and 4 are 0, 12, 24, .... The least common multiple (LCM) of two numbers is the smallest number (not zero) that is a multiple of both.
The greatest common factor (GCF) is the greatest factor that is common to two or more numbers (they share it).
The greatest common factor of two (or more) numbers is the product of all the prime factors the numbers have in common.