Pharo uses an Image. The image basically is the snapshot of your memory contents when you use Pharo.
Upon startup this image is loaded from the image-file into memory and Pharo starts to run. The inverse happens when you save (snapshot) your session: the current state/memory is saved to the .image
file. That includes all tools opened in the current session, all running processes and all live objects.
This has nothing to do with Fuel, which is a separate object serialization library.