“Plugins are ways to extend and add to the functionality that already exists in WordPress.
The core of WordPress is designed to be lean and lightweight, to maximize flexibility and minimize code bloat. Plugins then offer custom functions and features so that each user can tailor their site to their specific needs. ”
You can find more info about WordPress plugins on WordPress Codex portal.
While working on a client websites I have developed a lot of custom WordPress plugins to suit the needs of my clients.
If you need a custom WordPress plugins developed for you feel free to contact me and I will be happy to help you.