Are Print-on-Demand (POD) and self-publishing the same?
No. Print-on-Demand (POD) is just a printing technique. It can be used for traditional publishing or self-publishing.
Similarly self-publishing can be done with offset printing or with Print-on-Demand (POD).
These terms are often used together and even interchangeably, because self publishers, with tight print budgets, are probably the ones who benefit from POD almost immediately and hence are one of the first to embrace POD.