The latest update to the Core Resource articles is the introduction to Visualforce. Intended mainly for developers who may not have had a formal introduction or would like to get a sense of the surface area of the force.com UI technology it should be a worthwhile read. Here's the abstract:
Force.com platform. The framework includes a tag-based markup language,
similar to HTML. Each Visualforce tag corresponds to a coarse or
fine-grained user interface component, such as a section of a page, or
a field. Visualforce boasts over 60 built-in components, and a
mechanism whereby developers can create their own components.
Visualforce uses the traditional model-view-controller (MVC)
paradigm, with the option to use auto-generated controllers for
database objects, providing simple and tight integration with the
database. You can write your own controllers, or extensions to
controllers, using Apex Code. Visualforce also provides AJAX
components, and embeds the formula expression language for action, data
and component binding interaction.
This article introduces Visualforce. It illustrates the major
areas of functionality, provides an example of the MVC paradigm in
action, shows how to include database integration, and demonstrates how
to create your own components.
If your curiousity has been sufficiently piqued, continue with the rest of the Introduction to Visualforce.