Content areas is a concept, and a type, ContentArea, in EPiServer CMS's API. Content areas are used to hold an ordered list of references to content (pages, blocks etc) within the same EPiServer site.
A content area property is a property of type PropertyContentArea. Its value type is ContentArea.
It's possible to render the contents in a content area in any way we like as developers. However, the standard rendering, using either the Property control or the HTML helper PropertyFor, renders each content item in the block using a partial renderer. A partial renderer is typically a user control in Web Forms and a partial view, optionally created by a controller, when using ASP.NET MVC.