collections & film rolls
A collection is a set of images matching a given selection criteria.
The most basic kind of collection is a film roll, which contains all of the images that have been imported from a specific folder on disk. Whenever you import images from the filesystem, those images are organized in a film roll whose name is derived from that of their parent folder.
You can easily construct other kinds of collection based on various image attributes (Exif data, filename, tags etc.) in the collections module. Multiple criteria can be logically combined to narrow or extend your collection.
darktable retains a list of the most recently used collections for quick access. These can be accessed from the recent collections module.