What are prime common factors?

Example: What are the common prime factors of 120 and 252?

Find the prime factorization of each number by breaking it down into the lowest possible numbers that can be multiplied together to make the original number.

120 = 2x60 = 2x2x30 = 2x2x2x15 = 2x2x2x3x5

252 = 2x126 = 2x2x63 = 2x2x3x21 = 2x2x3x3x7

Notice that there are 2 2's and a 3 in each factorization: the common prime factors are 2, 2, and 3.