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. hook_views_data_alter() may also be used to modify existing data, changing other module's handlers or adding handlers to other module's tables. Views also allows modules to create new display types, style types, row styles, argument default handlers and argument validators via hook_views_handlers() and 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_views_data, hook_views_data_alter(), hook_views_plugins(), hook_views_handlers(), as well as any other hooks and subsidiary data that will only be used by your module when Views is active. All handlers and plugins provided by your module should be in separate .inc files. 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.

hook_views_api()

In order for your files to be included, your module must first implement hook_views_api() in the main .module file. This module should return array of information. The following items may be returned:

api: This must appear; it should be the oldest API version that your module can work with. If Views is currently running an older version of the API, it will ignore your module's views integration. This is a good thing, as it will prevent code crashes, at the expense of your module's functionality disappearing.
You may find the current Views API version by calling views_api_version() which is implemented at the top of views.module. This version numbering starts at 2.0. Every time changes are made to the Views handlers and plugins or other aspects of the Views API, the number will tick up (by either .001, .01 .1 or 1 depending upon how major the changes are). Note that views_api_version() was introduced in Views 2.0-rc2 and may not exist prior to that version. You may use function_exists() to test to see if this function is there.
Often these versions are basically compatible with each other and Views won't care if your module implements 2.000, 2.001, 2.002, etc. Your module can request that it won't work with any version older than a given version, however. Views will determine, itself, if a newer version will work.
path: If your *.views*.inc files are not in the same directory as the .module file, then return the full path here. You should probably use something like drupal_get_path('module', 'yourmodulename') . '/includepath'.