1. Getting to know S2
1.1. Common Terms
A journal view is the name for the different parts of a LiveJournal account which a user can create or customize a layout for. Someone's journal itself is called the "Recent" view, meaning that it shows all of that user's recent entries. The most popular view is the "Friends" view, with which a user can read all of their "friends'" updates.
S2 uses the concept of "layers" to provide different levels of customization - each layer in a style deals with a different aspect of the style, ranging from color to translation to HTML options and settings. A complete style is the result of putting its different layers together for use.
The core layer is the foundation of all styles. The core layer defines all of the functions and classes available to the lower layers. The core layer itself is fully functional as well; someone could use just the core definitions for each journal view to create a working (yet rather bland) style. The core layer is mostly just a base to work from, a guide to help form a complete style that helps by patching holes where necessary.
Core - Internationalization
The core internationalization layer provides all of the text information to help display someone's journal in a different language. This layer, like the core layer, is only really a base or a guide to create new internationalization layers from.
A layout layer is where the real customization starts. To draw a loose comparison to the old style system, a layout layer can be regarded as an S1 style except that the S2 layout covers each journal view, unlike the s1 style that only dealt with a single view. A layout layer, much like the core layer, defines its own set of properties that change the appearance and behavior of the layout.
An internationalization layer provides all of the text information that pertains to a specific layout layer, so that users can select which language their journal should display in. A complete style uses text from both an internationalization layer and the core internationalization layer.
A theme layer uses the color properties available in a layout layer to create a unique look for the style. To draw another loose comparison to the old style system, a theme layer is much like a color theme, except that a theme is only unique to one layout, unlike its S1 counterpart, which only worked for all styles.
A user layer is the final layer where most people's options will be set. Few people will create a user layer by hand, because a user layer is mostly handled by the customization wizards. To draw a final comparison to the old style system, you can think of a user layer as the equivalent to the Overrides box for S1, except there is a lot less headache involved in creating a user layer.
I'm on my way!