The conditional probability P(A | B) is the probability that an event A occurs given that an event B has occurred. It equals P(A and B) / P(B), with P(B) non-zero. Two events are independent when knowing B does not change the probability of A.
Example
Among 40 customers who received an offer, 10 buy: P(purchase | offer) = 10 / 40 = 0.25.