A quick note about copyright and your site
From the start let's make one thing clear - you own your site, the content of the site is yours and does not belong to anyone else. There is an obligation to maintain the copyright notice within Joomla's PHP source code but you are not obliged to allocate the copyright of your site to anyone but you in the publicly visible page footer
Now lets get on with answering the question at hand:
Essentially there are three ways to change this text.
1. Within your template's index.php file.
2. Within the /includes/footer.php file.
3. Within the language files.
A little explanation of how these methods are inter-related:
Each template set lives in its own sub-folder within /templates/
A template consists of:
an index.php (not to be confused with the scripts index.php in the installation root.
a css file usually css_template.css contained within /templates/
additionally image files within /templates/
/includes/footer.php is part of the core distribution of the script.
Your template's index.php file 'includes' - programmer speak for merges - footer.php into the final output of the page.
footer.php itself retrieves the Copyright text from the languages files.
There are pros and cons to either of these strategies, which I will explain as I go.
1. Editing your template's index.php so as not to include includes/footer.php and to simply place your text directly into the template.
Pros: Quick easy and efficient. It will also survive any upgrade of the core installation.
Cons: If you change templates, or use multiple templates you will have to replicate your edits throughout multiple templates.
Find and remove this line from the file /templates/
2. Editing includes/footer.php so it does not retrieve the copyright info from the language files. Instead enter your own details here.
Pros: Again fairly quick and easy - possibly more efficient as the info will be available throughout multiple template sets.
Cons: The includes/footer.php file is part of the core distribution - it gets overwritten when you upgrade your favourite CMS.
Find and remove these lines, or similar from the /includes/footer.php file, and replace with your own html or php.
3. Editing the language files directly - so that footer.php retrieves your info.
Pros: does the job
Cons: less transparent to anyone who takes over from you as editor. Also prone to being overwritten when you upgrade your favourite CMS.
Specifics: edit the file /languages/english.php
Some people have suggested moving the includes/footer.php file, for example into your template's folder, and editing the template to include this file.
Pros: still fairly quick and easy, and will survive an upgrade of the core CMS.
Cons: only compatible with templates that have been edited to work this way. Other templates may cause an error when the file is not found in its default location.
Good news for the future:
In upcoming version 4.5.3 the footer is a module.
No need to hack any code any longer.
This is not my work - I have just aggregated and summarised the work of others.
minor edits by mod. 10/28/06
DISCUSSION of this thread can be done here: http://forum.joomla.org/index.php/topic,49646.0.html