WP_Post is used to contain post objects stored by the database and is returned by functions such as get_post.
The data contained in the object are variables. Examples are ID, post_author_post_type, post_title, post_date and many more.
The WP_Post class is used to contain post objects stored by the database and is returned by functions such as get_post.
Wordpress introduced the WP_Post object which allowed post meta to be fetched using virtual properties. So if you aren't familiar with classes or virtual properties, all you need to know is the meta key for your post meta as if it were a property of the post object.