Who needs user-defined exceptions

Question:
Oracle defines a number of exceptions for me, like no_data_found. Why would I ever need to or want to define my own exceptions?
Answer:
Oracle does, indeed, pre-define a number of exceptions. Many of these may be found inside the standard package of PL/SQL, one of the two default packages in the PL/SQL language (the other being dbms_standard). You can view these exception definitions by looking at the stdspec.sql file in the $ORACLE_HOME/Rdbms/Admin directory of an Oracle10g installation.
Here is a portion of that file showing the declarations of a few such exceptions:
/********** Predefined exceptions **********/

CURSOR_ALREADY_OPEN exception;
pragma EXCEPTION_INIT(CURSOR_ALREADY_OPEN, '-6511');

DUP_VAL_ON_INDEX exception;
pragma EXCEPTION_INIT(DUP_VAL_ON_INDEX, '-0001');

TIMEOUT_ON_RESOURCE exception;
pragma EXCEPTION_INIT(TIMEOUT_ON_RESOURCE, '-0051');

In this code, Oracle declares an exception and then associates that named exception with an error code using the exception_init pragma.
You can declare exceptions just like Oracle does in the above code fragment; you can also raise application-specific exceptions "on the fly" with the raise_application_error built-in.
You will generally want to do this in order to improve the readability and maintainability of your code. Let's look at two examples: handling exceptions raised by Oracle and raising application-specific exceptions.