How does Java define its own exception?

import java.io.*;

// A Java application to demonstrate making your own Exception class// This program catches the exception when the word "client" is// entered incorrectly.

public class TestException{  static String s = "";  

//--------------------------------------------------------  public static void main (String args[])  {    InputStreamReader is = new InputStreamReader(System.in);    BufferedReader buf = new BufferedReader(is);    System.out.println("Enter the word you cannot spell: ");    try    {      s = buf.readLine();    }    catch (IOException e)    {      System.out.println("IOException was " + e.getMessage());    }    

    try    {      checkSpelling();   // this method throws SpellException    }    catch (SpellException se)   // but it is caught here    {      System.out.println("Spell exception was: " + se.getError());    }  } // end main  

//----------------------------------------------------------// Check spelling of typed in word. Throw exception if wrong.// Note how this method specifies that it throws such and such // exception. Does not have to be caught here.

  private static void checkSpelling() throws SpellException  {    if (s.equalsIgnoreCase("client"))      System.out.println("OK");    else      throw new SpellException("Cannot spell client");   }

}  // end main class

//***********************************************// Custom exception class that descends from Java's Exception class.

class SpellException extends Exception{  String mistake;

//----------------------------------------------// Default constructor - initializes instance variable to unknown

  public SpellException()  {    super();             // call superclass constructor    mistake = "unknown";  }  

//-----------------------------------------------// Constructor receives some kind of message that is saved in an instance variable.

  public SpellException(String err)  {    super(err);     // call super class constructor    mistake = err;  // save message  }  

//------------------------------------------------  // public method, callable by exception catcher. It returns the error message.

  public String getError()  {    return mistake;  }}