If you’ve ever wondered how wordpress plugins work, you’ll be surprised to see how simple they are.
Let’s create a plugin (I’ll call it “capitalizer”) that just modifies the text of the blog posts.
In order to do that, create a PHP file [wordpress root]/wp-content/plugins/capitalizer.php
and activate it from the plugins admin page. WordPress recognizes them automatically from that directory and reads the name and description from the comment.
Done ! Your posts will be all upper case from now.
That was only a minimal example in order to show how to get started. Look at the WordPress Plugin API documentation for further information or find some books on amazon, like this one