Invoking the Loop on pages which aren’t within WordPress’ basic hierarchy

I’m trying to generate a newsletter from posts in different categories, and have decided that the easiest way is to just make a template page and have WP fill in the new posts just like it does on my homepage.

But I can’t figure out how to get WordPress to execute a loop wherever you want it to

I.E. not on a page which would usually be home or a single post or an archive.

Does anyone know any way of doing this? It must be similar to when people put a list of latest blog posts on a home page which isn’t within WordPress’ directory.

Thanks very much everyone ??