Objects are not serialised and stored in files, they exist (only) as the fundamental entity of the OS. These objects are not read/saved from disk, they exist until destroyed and are managed transparently by the OS - similar to the manner to which virtual memory pages are transparently managed by Unix/Linux.
Objects represent reasonably high level entities, perhaps images, sounds, etc, but probably not individual integers or strings. Objects may reference ("contain") other objects.
Objects are strongly typed and implement common sets of abilities. All picture objects can show themselves, duplicate themselves, etc.