How are common denominator and common multiples alike and different

A common denominator is when the denominators (bottom number) of two fractions are the same. Fractions must have a common denominator to be added or subtracted.

A common multiple is when two numbers can be multiplying to the same number. Ex) A common multiple of 3 & 4 is 12

A common multiple of 6 & 2 is 18

You would need to know the common multiples of numbers in order to make fractions have a common denominator.