CMS is big part of the system allowing users to modify their own content.
This is usually performed through control panel and varies per pack. The pack provides user with abilities to view her objects (whatever those are: property, documents, products, orders etc.), create, delete, update or perform any other action through user interface.
This is performed through control panel and is common for every website. Control panel allows a user to edit her website on-spot, which will take her to her website and then she may alter resources which are visible on a page: edit titles, upload pictures, change text etc. This is on-spot CMS which operates directly on a website page and is simple as click and change - easy to use but with limited functions. Any advanced changes outside of on-spot CMS can also be done through control panel CMS editor, which includes adding pages, modifying templates, uploading files and more. And if control panel interface does not allow required changes to be made then those changes can be dose by support.