Joomla Components are the main Joomla extensions, which provide vital functionality for every Joomla website. They are usually displayed in the center of the page.
Joomla Components define the most vital parts of Joomla page functionality. While a Joomla site may exist without modules or plugins, it could never work without components.
Some components are hard-coded into Joomla, like com_content, which provides the basic Article functionality, or com_users which provides the interface into managing Joomla users. The com_installer is the component responsible to allow for new extension installations.
If you are inexperienced in developing Joomla extensions, you may start with modules or plugins, before getting into components.
For the more experienced, you can read how a component works, here.
To start creating your own component, you can go here.
