A Collection is a virtual grouping of photos or videos, while a Collection Set is a folder that organizes multiple Collections and other output items like prints or web galleries. The key difference is that Collection Sets are containers for other collections, providing a higher level of organization, whereas Collections hold the actual photos. You can create Collection Sets inside other Collection Sets to build a hierarchical structure.
Collections are virtual groupings of photos that don't move the files from their original location, making them a flexible way to manage your workflow. You can also create automated "smart collections" that update automatically based on criteria you set
Working with smart collections.
Create a smart collection: Click the plus icon (+) next to "Collections" and select "Create Smart Collection".
Set criteria: Choose rules to automatically add photos to the collection, such as by rating, camera type, or date. For example, you could create a smart collection for all photos with a rating of 4 stars or higher from 2023.
See results instantly: The smart collection will automatically populate with all the photos that match your criteria
Organize collections: Create "collection sets" (folders) by clicking the plus icon and choosing "Create Collection Set." You can then drag and drop your collections into these sets.
Key concepts.
Collections vs. folders: Collections are virtual albums that group photos from different folders. They do not duplicate files on your hard drive, making them a non-destructive way to organize your work.
Quick Collection: A temporary holding area for photos you want to quickly group. You can save a Quick Collection as a new, permanent collection at any time.
Target collection: A specific collection that you can add photos to quickly by using the B key, without having to drag and drop them.
Why use collections in Lightroom Classic?
So you can have the same photo of a cat in 10 different collections. It doesn't matter to Lightroom you can put that photo into every collection or just one collection or no collections.
Should I use collections or folders in Lightroom?
Collections are different than folders. They can contain photos from many folders. They are simply a list of photos, which LR can use for specific tasks. The only time I use collections is to produce things like slideshows or photo books or similar