PhD is an advanced degree which has a very clear objective: to prepare students for their future research career. PhD programs are open to students with Master’s degree, but in some countries (such as the United States) it is possible to apply for PhD programs after your undergraduate degree. It usually takes between 3 and 7 years to complete your PhD studies. In most cases it will take more than 5 years, especially if you study in the United States, where you have to complete several classes and obtain you Master's degree before starting to write you final thesis.
PhD prorgams are typiccaly very intensive: students have to choose advanced classes as well as to think about their reserach project. Moreover, sudents often work as research or teaching assistants in the university which help to raise some money as well as to get some expirience, which could be valuable for their own projects.
However, career in academia is not the only option which is available after your doctoral degree. While 5 or seven additional years spent on research and studying may seem like a disadvantage for your career in business, some advanced skills acquired during these years may be valuable for employers. Some graduates may choose a career in the research departments of private corporations (after PhD in Biology) or in international organizations such as World Bank (after PhD in Economics).
Admission to PhD programs is typically very competitive: prospective students have to demonstrate excellent academic abilities (your undergraduate and graduate transcripts should be very solid as well as your test results), but the most important piece of your application is your statement of purpose or research proposal. You have to present your research project and explain why it is good enough to be realized. While PhD programs train you to become a researcher, the potential quality of your research is the most important criteria for an admission committee. Prospective students are ofter required to provide the results of the standardized tests, such as GRE.