Conditional probability measures the probability of an event occurring given the fact that another event has already occurred. For two events A and B, conditional probability is denoted by P(A | B), which is read as the probability of A given that B has already occurred. Conditional probability can be solved for using the rule P(A | B) = P(A, B) / P(B), where P(A, B) is the probability of both events occurring, and P(B) is the marginal probability of only B occurring.