The GCD (also called GCF) of two things is the biggest thing that
goes into both evenly. So, for two terms, it's the
biggest *term* that goes into both evenly. Its coefficient
(that's the number in front) will be the GCD of the coefficients
of the two terms you're given. The exponent on each variable will
be the lesser of its exponents in the two terms you're given. (If
some variable doesn't exist in *both* terms you're given,
then is doesn't appear in your answer either.)

For example, suppose you want to find the GCD of *b*
and *a*^{4}*b*^{2}*c*. The
coefficients of both these terms are 1 (remember, if there's
nothing in front, it's the same as if there's a ‘1’),
so the coefficient of the answer is also 1. Since *a*
and *c* don't exist in the first term we were given, they
don't appear in the answer. The letter that *does* appear
is *b*. Its exponent in the first term is 1 (remember, having
no exponent is the same as having exponent 1), and its exponent in
the second term is 2. The lesser of these is 1, so the answer is
1*b*^{1}. This is the same as just *b*, so
that's our answer.

Remember, having *no* coefficient (or exponent) is the
same as having a coefficient (or exponent) of 1 (*not*
0).