You've probably seen pocketknives with interchangeable blades or other tools (saws, forks, corkscrews, you name it). Plugins are the WordPress equivalent of those. You just choose the ones you want to install. Plugins can simply provide new features or they can change the way that WordPress works. Whether you choose a plugin to jazz up your WordPress blog or to hot-rod your whole Web site, you can do it without having to know anything about the underlying software code.
For example, if you have a WordPress blog and you want to add complex, fancy content (such as video), you can install a plugin to handle that content. If you have a WordPress Web site for your business and you want to create a form for visitors to use when they contact you, multiple plugins can do that job.
Core plugins (or canonical plugins) are new to WordPress. Each one delivers a set of features approved by the contributing developers at WordPress who wrote the main WordPress software. They're intended to provide the most requested features that would be useful to the majority of WordPress users. Core plugins seek to keep those features closely integrated with WordPress, resulting in a streamlined integration that makes them easy to use. Currently, only two core plugins are included in the WordPress software by default: