Views allows modules to describe their tables relationships to each other, as well as fields, filters, sort criteria and arguments via hook_views_data(). Whenever Views deems it necessary, this hook is called, the data aggregated together and cached. Views also allows modules to create new display types, style types, row styles, argument default handlers and argument validators via hook_views_plugins(). These hooks are kept in a file named MODULENAME.views.inc. This file is automatically included upon need, so there is no need to try and include this in hook_init or any other method of including .inc files. This file should store hook_data, all handlers and plugins, as well as any other hooks and subsidiary data that will only be used by your module when Views is active. There are two similar files, MODULENAME.views_default.inc and MODULENAME.views_convert.inc which contain default views and views 1 to views 2 convert helpers, respectively.