Hierarchical, sequential and pyramidal coding schemes.
Coding can be seqential (one pass) or progressive (multipass at increasing levels of detail) Progressive can result in better compression. eg 0.72 bits per pixel compared to 0 .74 for arithmetric entropy coding.

Hierarchical coding involves multiple versions of same image at increasingly higher resolutions. Each stage is used as the prediction for the next stage and it is the prediction error that is encoded. Sometimes called pyramidal coding. Usually less efficient than sequential coding.

Fig 1.5 shows how the four JPEG modes can be coupled. [2]

All JPEG compliant decoders must be able to handle the baseline system, ie sequential DCT.