The integral test is a method used in calculus to determine the convergence or divergence of an infinite series. It compares the series to an improper integral. If the integral of a function is finite, the series converges; if the integral is infinite, the series diverges. This test is applicable when the function is positive, continuous, and decreasing. To apply the integral test, one typically considers a series of the form ∑a_n, where a_n = f(n) for a positive, continuous, and decreasing function f(x). By evaluating the improper integral from a certain point to infinity, one can draw conclusions about the behavior of the series.