A sealed class in C# is a special type of class that prevents other classes from inheriting from it. When a class is declared as sealed, it means that it is the final implementation of that class and no other class can inherit from it. This is different from regular classes, which can be inherited by other classes to extend their functionality.

Sealed classes are typically used when a class needs to provide a complete implementation of a certain functionality or behavior and it is not designed to be modified or extended by other classes. This is because sealed classes provide a level of security and stability by preventing unwanted changes to their implementation.

Sealed classes also provide performance benefits by allowing the compiler to optimize the code more efficiently. Since a sealed class cannot be inherited, the compiler can make certain assumptions about the class's behavior, which can lead to faster and more efficient code execution.

To declare a class as sealed in C#, you simply add the sealed keyword to the class definition. Once a class is declared as sealed, it cannot be inherited by any other class.

In summary, sealed classes in C# are a useful tool for creating stable and efficient code by preventing unwanted changes to the class's implementation and allowing the compiler to optimize the code more efficiently.