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 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.