Limb contains the following directories in its root directory:

+-LIMB_DIR 
  |
  +-core (here all Limb classes are stored except test cases)
  |
  +-design (site appearance: images, templates, css-files etc)
  |
  +-var (PHP compiled templates, media files, log 
  | and cache file are stored here)
  |
  +-tests (Simpletest cases)

Project file structure should be similar to Limb. This is required by file path resolving rules.

Now we can examine the directory structure in more detail.

+-LIMB_DIR/core
  |
  +-actions (all classes that implement site functionality 
  | are stored here. The action classes for site_objects 
  | are usually kept in directories named specifically to 
  | simplify browsing, e.g. create_article_action, edit_article_action 
  | and delete_article_action are good candidates to be stored in the
  | Limb/core/actions/article directory, this is not mandatory 
  | yet it helps to grow familiar with Limb)
  |
  +-controllers (site objects controllers are placed here)
  |
  +-datasource (fetching helper classes can be found here. 
  | Limb templates are active and their activity mostly 
  | implemented by datasources)
  |
  +-db_tables (db_tables are classes representing specific 
  | tables in the data base. These classes have methods for 
  | type-casting, fetching/creating/editing/deleting rows. 
  | They're also useful for cascade deleting since some db 
  | drivers don't support this feature(e.g. MySQL). 
  | In the nearest future we are planning to use db tables to 
  | create specific db schema automatically)
  |
  +-filters (intercepting filters are here).
  |
  +-lib (this a Limb utility library)
  |
  +-locale (ini-files for locale configurations can be found here)
  |
  +-model (Limb keeps model classes mostly here. However one can 
  | find also audit, statistics, search, response related 
  | stuff here too(subject to be placed somewhere else). 
  |
  +-request (request and response classes. action->perform() is 
  | responsible to return some response) 
  |
  +-settings (settings ini-files are kept here)
  |
  +-strings (ini-files with localized strings)
  |
  +-template (WACT template mechanism with Limb specific tags 
  | and components)
  |
  +-tree (classes to deal with hierarchic structures)

LIMB_DIR/core/model directory

+-LIMB_DIR/core/model    
  |
  +-search (search functionality classes)
  |
  +-shop (a simple yet robust implementation of the shopping 
  | cart)
  |
  +-site_objects (site_objects classes)
  |
  +-stats (statistics and user audit related classes 
    are kept here. You can also find here stats 
    and audit report classes)

LIMB_DIR/core/template directory

+-LIMB_DIR/core/templates    
  |
  +-compiler (compiler classes)
  |
  +-components (runtime components classes)
  |
  +-fileschemes (file name resolving classes)
  |
  +-tags (tags files)
    |
    +-actions (tags to display JIP-actions)
    |
    +-cart (helper tags for shopping cart)
    |
    +-core (some core WACT original tags and a lot 
    | of Limb specific ones(probably some of 
    | these should be moved to somewhere else)
    |
    +-datasource (datasource tag)
    |
    +-dev (tags to view templates sources)
    |
    +-fetch (template's fetching tags)
    |
    +-form (tags to work with forms. 
    | Includes selectors, buttons, inputs, etc)
    |
    +-grid (tags to output tabular data,
    | substitute for WACT list tags)
    |
    +-hint (tags to display small popups with legend)
    |
    +-locale (data l10n tags)
    |
    +-metadata (meta specific tags)
    |
    +-pager (tag for tabular data paginating)
    |
    +-phpbb (some helper tags to use Limb integrated version of phpbb)
    |
    +-poll (poll tags)
    |
    +-print (outputs link to print version of object 
    | if it supports print_version action)
    |
    +-status (document flow tags)
    |
    +-user (some helper tags to render user 
      specific attributes, like login, name, etc)