Core answer
Start here. This is the direct spoken answer to practice first.
Overview
Class and object are often used interchangeably in casual discussion, but an interview answer should distinguish the type definition from a concrete runtime instance.
A class is a C# type definition that describes the data and behavior its instances can have, while an object is a concrete runtime instance of a type. For example, Order can be a class with an Id, item state, and a Confirm method; new Order(...) creates one particular order object with its own state and identity. Many objects can be created from the same class, and they share the class's behavior while holding separate instance data. Static members belong to the type itself rather than to one object.