An excerpt from http://www.sitepoint.com/making-your-wordpress-plugins-developer-friendly/ , by Tim Carr
WordPress plugins are typically used by two groups of people: WordPress web site owners, and developers. It’s important to meet the needs of both sets of users.
Developers may be people with a basic level of WordPress / PHP knowledge, who want to tinker with your plugin. They may also be experienced WordPress developers who want to use your plugin to provide specific functionality to a client’s web site.
If you’d like an introduction to plugin development for WordPress, Tim Smith’s article WordPress Plugin Development is worth a read.