There are 23000 plus Plugins in the Repository segregated by category, which are downloadable or are links to other Plugin repositories. Each downloadable Plugin showcased generally documents their Description, Installation, FAQ, Screenshots, Other Notes, Changelog, Stats, Support, Reviews, Developers. You can manage your installed Plugins (install/ uninstall/ upgrade) using the Plugin Section in WordPress Administration Panel (a WordPress Management Panel) or do it manually.
Custom Plugins can be developed to have a new set of features either not available or that is needed for your needs. The Plugin Development documentation has three sections – Writing a Plugin, Plugin API, and Plugin Resources. The Plugin Development section documents predefined coding standards/ methods for each new Plugin from naming, file code commenting, licensing, usage of Hooks (connects and calls the Functions in Plugin), usage of template tags (adds Plugin functionality), save your Plugin data or options, or adding your Plugin Menus to the Administration Panel for installation/ uninstallation/ updating. The Plugin API gives developers the Hooks, Actions, and Filters needed to connect WordPress to call your functions for use. They provide features like addition/ modification of new functions, new actions, display, database data, or sending an email.
Plugin Developer Center
Each Plugin is managed from the developer center that manages, documents, and follows organized methods for addition of new Plugins by community developers. Plugins are maintained by their developers in version control – subversion (SVN) of specific trunk/ folder in WordPress site. The trunk files maintained are Plugin files and their version tag in readme.txt (text) file. The tag file contains details of Authors, Version, Tags (as in categories), Plugin Name, Author, and Plugin Homepages, Last updated time, Creation time, and Time of first check in.
WordPress Support Page: http://codex.wordpress.org/Main_Page
Word Press Plugin Page: http://wordpress.org/extend/plugins/
WordPress Managing Plugins: http://codex.wordpress.org/Managing_Plugins
WordPress Plugin Development: