Whats the difference between a 'page', a 'panel', and a 'handler'? I am porting a site over and looking through the code, and in a module there are folders for pages, handlers, and plugins/layouts (containing panels layouts) . The pages and handler folders contain .inc files. They are exports of pages and panels etc. However, what is the definitive difference between a handler, panel, and page?
Thanks
If you navigate to admin/structure/panels. It gives a short description.

Panel page

Panel pages can be used as landing pages. They have a URL path, accept arguments and can have menu entries.

Custom content
Custom content panes are basic HTML you enter that can be reused in all of your panels.