data directory, with text files as topics. System for documentation, Main to store user pages and site-wide preferences, Trash for the topic recycle bin. Sandbox/TWiki/SubWeb.SubWebTopic topic: System.DefaultPreferences default settings
Main.SitePreferences site-wide settings inherits from and overrides settings in System.DefaultPreferences
Sandbox.WebPreferences inherits from and overrides settings in Main.SitePreferences
Sandbox/TestWeb.WebPreferences inherits from and overrides settings in Sandbox.WebPreferences
Sandbox/TWiki/SubWeb.WebPreferences inherits from and overrides settings in Sandbox/TestWeb.WebPreferences
Sandbox/TWiki/SubWeb.SubWebTopic inherits from and overrides settings in Sandbox/TWiki/SubWeb.WebPreferences
Main.SomeUserName - points to it and would need updating (unless the macro style %USERSWEB%.SomeUserName, is used throughout). This potentially large change can be performed automatically if you rename the web from the Tools section of WebPreferences, as described above.
{SystemWebName}, {UsersWebName} and/or {LocalSitePreferences} settings in the configuration using the configure interface.