Guides is the knowledge base and you can consider this a library of knowledge you can provide your customers for your business.
Collections are the top most layer of a knowledge base. They are also optional in a way. You can think of collections as essentially books of knowledge around a subject. They are great for different products which may need multiple categories.
Note: If you create just one collection, then it will become the default.
Categories are used to organize articles under a common theme and can be thought of as chapters in a book. Articles may belong to multiple categories.
Articles are like pages in a book that allow you to document knowledge that can save time by allowing customers to read and get answers to their questions by supporting themselves before ever needing to contact support. This can be how-to guides, tutorials, documentation, and frequently asked questions.
Imagine you are Dunder Mifflin and provide support for multiple products. This is a good starter structure setup for that business.
Collections: Paper, Printers
Printer Categories: Inkjet printers, Laserjet printers
Inkjet Printer Articles: Setting up drivers, How to replace the ink cartridge