The two theorems are basically use the same principles. The difference is:
-In remainder theorem, in a polynomial f(x), when you divide the f(x) by the binomial (a-x), the remainder is equal to the f(a). This saves you from doing the long division as you only substitute the x for a.
-The factor theorem, on the other hand, tells us that the binomial (x-a) is the factor of the polynomial f(x) when after the substitution for x with a gives a zero.