A key task in the scope of Content Management, is Content Modeling - this is when you get raw content to fit into some common model of types and elements. Once defined, the types and their elements fit into a hierarchy (aka - Taxonomy or "Content Tree"). To break this down a little further, let's first take a look a "types".

Content Types
At the foundation of most CMS systems is the concept of types - Content Types (aka - Content Objects, Classes). This enables developers/administrators to structure content according to it's format/function -- examples of types might be "articles", "blog post", "FAQ", "image gallery", "product description", etc..

The way that types are defined varies greatly between Content Management Systems. A basic CMS may provide some pre-defined types with a few options, where as a more robust CMS will allow for custom types, and are usually defined/managed using database tables, templates or controls.

Content Elements
When we breakdown each Content Type it contains Content Elements.  The elements are the properties of the Content Type. These "elements" may be text, images, HTML blocks or rich-media. It makes sense that we label the elements in ways that tie into their layout or parent Content Type. For example, in the "article" type there might be a "Heading", "Author", "Date" and "Body".

Content Schema -- The Hierarchy
There is currently not a standard defining how a CMS needs to implement it's structure. I'm not referring to topical classification (ie; categories, tags), but rather the concept of the "Content Tree" - a clear method of mapping the content into a hierarchy. If you'd like to read more about Information Architecture and classification - Clay Shirky once a wrote a great article on it.

When Content Modeling Happen?
Content Modeling occurs at the configuration level. This happens after development (unless your developing your own CMS), and before Content Authoring. The editors of the Content are usually not involved in this step, but rather the developer or adminstrator responsible for deployment of the CMS. In general, I think there are three steps to take during the Content Modeling process:

1. Identify Users: Not limited to audience, this is everyone involved. Visitors, readers, editors and admins.
   
2. Identify Content Types: As described above, breakdown your content into types and elements within those types.
   
3. Determine Relationships: "Articles" is most likely related to "Events". This step and the prior will be iterative.

Best of all, a good content model will ultimately improve your sites' business model.