In C#, a variable that holds a value is called a value type. Value types include basic data types like integers, floats, and doubles, among others. Value types in C# cannot be assigned a null value. However, nullable types allow you to assign a null value to a value type.

A nullable type is created by adding a '?' symbol after the value type. This allows the variable to hold either a valid value of the specified value type or a null value. Nullable types are commonly used in situations where a database column may contain null values.

When assigning a value to a nullable type, you can assign a valid value of the specified value type or null. It is important to note that when performing operations on nullable types, you need to be aware of whether the value is null or not. If the value is null, any operation performed on it will result in a null value.

To handle null values in nullable types, you can use the '??' operator to provide a default value. This operator checks whether the value is null and, if it is, assigns the default value instead.

In summary, nullable types in C# allow value types to represent a null value, which can be useful in certain situations. They are created by adding a '?' symbol after the value type, and you can assign a valid value or null to them. To handle null values, you can use the '??' operator to provide a default value.