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What is a "package"?

According to the PL/SQL User's Guide and Reference, "A package is a schema object that groups logically related PL/SQL types, items and subprograms."

But I believe a package is far more than just a way of logically grouping objects together.

Before I digress, let's very briefly understand what a package is. It's probably easiest if you take a look at Oracle's documentation, which has a good description of packages and some examples:

PL/SQL User's Guide and Reference, Chapter 9: PL/SQL Packages:
http://download-west.oracle.com/docs/cd/B10501_01/appdev.920/a96624.pdf

Put simply, packages have two parts, the specification and the body.

The specification is the interface into the package. It describes all the types, variables, procedures (etc) within the package. Ideally the specification does not change.

The body has all the implementation. Inside the body you would actually write the procedures, assign the values, and have all the details and logic behind the package. Often (unless you are the developer), you can treat the body as a "black box."

As always, I'll refer you to Dan Morgan to understand the syntax of Oracle PL/SQL Packages:
http://www.psoug.org/reference/packages.html

Back to the story.

According to one of my favourite PL/SQL Authors Connor McDonald, "The main reason packages are not more widely adopted is that users are unaware of the benefits they offer."

Beyond the known, obvious advantage of grouping together related items, which is great for organising your code and "modularity", what are these benefits of which we are speaking?

1. Objects don't get invalidated when you makes changes to the body. That saves a lot of recompilation and makes changing the implementation much more painless. You will still have to recompile if you change the specification, but that's not something you should be doing very often.

2. You can "overload" subprograms (procedures/functions). You can have several subprograms with the same name, but with a different number of parameters, or different types. That is another thing that makes implementation changes more painless because you can keep legacy code if you like. You can also see the extra flexibility that offers developers.

3. You can have persistent variables throughout a session without storing anything in a database table. Packages can have variables and constants that are initialised when the packages is first used within a session, and then they are available for the remainder of the session for all future references to anything within that package. That comes in very handy.

4. Speaking of initialisation, being able to call a procedure automatically the first time a package is used within a session can also come in very handy.

5. You can take advantage of "encapsulation." In essence, you can hide the implementation details from users but still give them all the information they need to use the package. Since they aren't aware of the details, that means you can change them with minimal impact or risk. Packages also support private subprograms and variables which are available only to other subprograms within the package, and remain completely hidden and inaccessible to anything outside the package.

6. You may notice some performance improvement when using packages. When you first use a package, the entire package may be loaded into memory, meaning fewer disk I/Os as you use the related items within.

I'm sure there are other advantages, but those are the ones I've noticed.

Let me close with links to my favourite two articles about Packages, from two of my favourite PL/SQL experts:
Connor McDonald's Chapter 2 from Mastering Oracle PL/SQL: Practical Solutions
http://www.oracle.com/technology/books/pdfs/sample2174ch2.pdf

Steven Feuerstein's article "Picking Your Packages" from May/June 2005 Oracle Magazine
http://www.oracle.com/technology/oramag/oracle/05-may/o35plsql.html

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Addition:
By the way, to use packages, just prefix the package name using dot notation

Check out PL/SQL Supplied Packages and Types Reference
http://download-west.oracle.com/docs/cd/B10501_01/appdev.920/a96612.pdf

It includes UTL_HTTP and DBMS_OUTPUT
http://thinkoracle.blogspot.com/2005/08/utlhttp.html
http://thinkoracle.blogspot.com/2005/10/dbmsoutputputline.html

But also check out UTL_FILE, DBMS_PIPE, DBMS_ALERT and the DBMS_XML stuff.