save_post is an action triggered whenever a post or page is created or updated, which could be from an import, post/page edit form, xmlrpc, or post by email. The data for the post is stored in $_POST, $_GET or the global $post_data, depending on how the post was edited. For example, quick edits use $_POST.
Since this action is triggered right after the post has been saved, you can easily access this post object by using get_post($post_id)
http://codex.wordpress.org/Plugin_API/Action_Reference/save_post