Create an Static Block from your CMS admin tab and show it in your theme.
In my case I wanted to call a Static Block from my theme to show up in the sidebar.
I edited my theme file called 2columns-left.phtml and then added the following code.
Please notice you need to replace the YourBlockId with your own Static Block Id.
<?php echo $this->getLayout()->createBlock('cms/block')->setBlockId('YourBlockId')->toHtml() ?>