All of these block types share most PL/SQL features so during this tutorial the features that apply to all block types will be grouped into single subjects.

As you can see there are several elements in the example that haven't been covered in the SQL tutorial, these elements are the PL/SQL extensions. They include:

Rules of block Structure

1. Every unit of PL/SQL must constitute a block. As a minimum there must be the delimiting words BEGIN and END around the executable statements.
2. SELECT statements within PL/SQL blocks are embedded SQL (an ANSI category). As such they must return one row only. SELECT statements that return no rows or more than one row will generate an error. If you want to deal with groups of rows you must place the returned data into a cursor. The INTO clause is mandatory for SELECT statements within PL/SQL blocks (which are not within a cursor definition), you must store the returned values from a SELECT.
3. If PL/SQL variables or objects are defined for use in a block then you must also have a DECLARE section.
4. If you include an EXCEPTION section the statements within it are only processed if the condition to which they refer occurs. Block execution is terminated after an exception handling routine is executed.
5. PL/SQL blocks may be nested, nesting can occur wherever an executable statement could be placed (including the EXCEPTION section).

Your first example in PL/SQL will be an anonymous block --that is a short program that is ran once, but that is neither named nor stored persistently in the database.

PL/SQL code is compiled by submitting it to SQL*Plus. Unless your program is an anonymous block, your errors will not be reported. Instead, SQL*Plus will display the message ``warning: procedure created with compilation errors''. You will then need to type:

to see your errors listed. If you do not understand the error message and you are using Oracle on UNIX, you may be able to get a more detailed description using the oerr utility, otherwise use Oracle's documentation (see References section).

Next, we will rewrite the anonymous block above as a procedure. Note that we now use the user function to greet the user.

Make sure you understand the changes made in the code: Once you have compiled the procedure, execute it using the EXEC command.

Generating Output on Screen

The built-in packages offer a number of ways to generate output from within your PL/SQL program. While updating a database table is, of course, a form of "output" from PL/SQL, this chapter shows you how to use two packages that explicitly generate output. UTL_FILE reads and writes information in server-side files, and DBMS_OUTPUT displays information to your screen.

Here is a very good chapter from O'Reilly which helps to understand the complete concept and usage of “Generating Output on Screen”  Click here to Read

First Page: Tutorial 1: Introduction to Oracle 9i:SQL, PLSQL. and SQL *Plus