A content management system is used to manage the creation and modification of entities, typically business objects (e.g, vehicles, assets, contracts), that are completely customizable.
When you create an entity, you decide what kind of structure each item of that entity will have. For example, if you're creating an entity for "vehicles", you might need a make, pictures and a reference number for each vehicle. This information is modeled using properties (rich-text, image, number...) and permissions. A property can also be a reference to another entity, which allows you to build complex data structures.