Trong lập trình hướng đối tượng, một lớp (class) là một tập hợp các dữ liệu và các phương thức (còn gọi là các hàm thành viên). Lớp là cơ sở để tạo để tạo ra các đối tượng.

In object-oriented programming, a class is a construct that is used to define a distinct type. The class is instantiated into instances of itself – referred to as class instances, class objects, instance objects or simply objects. A class defines constituent members that enable its instances to have state and behavior. Data field members (member variables or instance variables) enable a class instance to maintain state. Other kinds of members, especially methods, enable the behavior of class instances. Classes therefore define the type of their instances. 

A class usually represents a noun, such as a person, place or thing, or something nominalized. For example, a "Banana" class would represent the properties and functionality of bananas in general. A single, particular banana would be an instance of the "Banana" class, an object of the type "Banana".