Complexity classes are categories used in computer science to classify problems based on the resources needed to solve them, such as time and space. For example, the class P includes problems that can be solved quickly (in polynomial time) by a computer, while NP consists of problems for which a solution can be verified quickly, even if finding that solution might take longer. Understanding complexity classes helps researchers determine how difficult a problem is and whether efficient algorithms exist. The famous P vs NP question asks if every problem whose solution can be quickly verified can also be quickly solved, a fundamental question in theoretical computer science.