The "Concentrate Questions and Answers" series helps students prepare for exams by discussing different types of mistakes in contracts. There are three main categories: common mistake, mutual mistake, and unilateral mistake. In common mistake, both parties make the same error about a key fact in the contract. Mutual mistake occurs when both parties have different intentions without realizing it. Unilateral mistake happens when only one party is mistaken, and the other party is aware of the error.