Start here. This is the direct spoken answer to practice first.
Overview
Class and object are related but not interchangeable: one defines a type, while the other is a runtime instance.
A class is a Java type definition that describes the fields and behavior its instances can have. An object is one concrete runtime instance of a class. For example, Order can define an identifier, item state, and a confirm method; each call to new Order(...) creates a particular order object with its own state and identity. A variable such as Order order holds a reference to an object, not the class definition itself. Static members belong to the class rather than to an individual object.