What they said. The word "serial" refers to the fact that the data bytes must be put into some standardized order to be written to a serial storage device, like a file output stream or serial bus. In practice, the raw bytes seldom suffice. For example, a memory address from the program that serializes the data structure may be invalid in the program that reconstructs the object from the stored data. So a protocol is required. There have been many, many standards and implementations over the years. I remember one from the mid 80's called XDR, but it was not the first.
