Photo CD
Developed in the early 90s by Kodak and Philips (who seems to have its hand in everything CD-related), photo CD is an implementation of CD-ROM extended architecture designed to hold photographic images. They technically use mode 2 form 1 of the CD-ROM XA architecture. Photo CDs are defined in the "orange book" specification.
When you send in film for processing to photo CD, the film is first developed normally. The developed and printed pictures are then scanned and converted to digital form, encoded into the photo CD format, and written to the CD. Writing the photos to the CD is done using a process that is basically the same as how CD-R works: a laser burns the information into the tracks of the CD.
After you have sent in film and created the first photo CD, it is possible to record additional films to the same disk. However, doing this means that the information is written in multiple sessions, and therefore a player that supports multiple sessions is required to access the disk properly.