The programs are somewhat interdisciplinary in their focus and ultimate goal. In most cases the PhD programs lead students to become somewhat of an expert in a selected subject matter, and become proficient at navigating and utilizing the research tools to prepare for the ultimate attainment of the PhD designation. Broad understanding and the role and scope of any specific program of study is ultimately achieved, and a level of competency in the particular selected field of study is advanced. Hence you will become an expert on that which you choose to study. Programs of study can vary in length.
Some studies require two years for completion, others can take a decade.
In most cases the criteria for entrance are similar, and most institutions require that you participate in an approved program of study. Minimum GPA requirements must be met. Both written and oral examinations must be taken, and there is often a residency requirement. Typically a PhD candidate must present to their graduate advisors a dissertation. It must represent original concepts in research, abilities for independent research, scholarly ability and original thought.