Resultant[, , var] computes the resultant of the polynomials and with respect to the variable var.
Resultant[, , var, Modulus->p] computes the resultant modulo the prime p.
The resultant of two polynomials and , both with leading coefficient one, is the product of all the differences between roots of the polynomials. The resultant is always a number or a polynomial.
See The Mathematica Book: Section 3.3.4.
See also: Subresultants, PolynomialGCD, Eliminate.