Plugin extra posts/pages

Hello,
I have a rather generic question. Please forgive me if not being clear enough.

Many plugins add their own custom content group. For example, besides posts and pages, installing a testimonials plugin creates a new group called ‘testimonials’. Inside that, one can create ‘posts’ of type ‘testimonial’ and then use them eg. via widgets or shortcodes. I guess this has something to do with the WordPress API, because they all look very similar.

That plugin might also create its own ‘page’. For example, there is now a new permalink called ‘mysite.com/testimonials’, which shows a page. However, this page is not accesible anywhere inside the admin interface.

My questions are:

1. What kind of object is this? Is it manageable via WordPress?
2. I would love to use the newly created ‘objects’, but I do not like the idea of extra pages being created, particularly if I do not have any kind of control on them. How can I remove what I don’t need?

Thank you