Conditionals are a type of sentence structure used in grammar that express a condition or set of conditions. They are often used to express hypothetical situations, such as 'IF I had more money, I would buy a car.' The most common type of conditional is the IF-THEN statement, which consists of an 'IF' clause followed by a 'THEN' clause. The three-letter acronym IFS stands for 'IF-THEN statements, ' which are the most common type of conditionals.