In these next few sections, we're going to cover plugins and widgets. Plugins and widgets are not a part of your theme; they are additional files that use WordPress-compatible, PHP code, and are installed separately into their own directories in your WordPress installation (again, not in your theme's directory). Once installed, they are available to be used with any theme that is also installed in your WordPress installation.
Even though plugins and widgets are not a part of your theme, you might have to prepare your theme to be compatible with them. We'll get to that in the following section.
WordPress has been built to be a lean, no-frills publishing platform. It simply means that with a little coding and PHP know-how, you can easily expand WordPress' capabilities to accommodate your site's specific needs. Plugins were developed so that even without a little coding and PHP know-how, users could add extra features and functionality to their WordPress site painlessly...