A website component is a set of function which can be installed onto a web page. An example of a website component is an image gallery. An image gallery renders images and it can be installed on any pages. Another sample of a component is a Contact Us form. A contact us form is usually placed on a contact us form web page.
There are predefined components that are commonly used and readily available. However, there are scenarios where companies would need new components or customize an existing one.