Based on the severity of the error, Exception classes are categorized into two groups: unchecked exceptions and checked exceptions. Unchecked exceptions are those that can be either handled or ignored. If the programmer ignores an unchecked exception, the program will terminate when such an error occurs. On the other hand, if a handler is provided for an unchecked exception, the result of the occurrence of such an error will depend on the code written in the exception handler. An example of an unchecked exception is the class RuntimeException (and its sub-classes), which is a sub-class of the class Exception. It is a very important class in Java programming.