Bachelor is a basic four years program. Master is two years above the bachelor, and Phd is two more years above the master. The requirements for Phd have to be either in teaching at the college or in research sponsored by a company or an organization. In US, the professional Dotorate degrees such as MD, OD, DDS, JD etc., it required minimum two years (60 semester or 90 quarter units), it by-passed the bachelor and master. Some schools offer students in an accelerated program, accepting the top students right after their graduation from high schools. So, six years is the minimum requirement for a professional Doctorate degree. Eight years for the Phd degree. Some people may argue that the professional Doctorate degree program required higher education such as bachelor, master or Phd in order to meet the entrance requirement - It is the supply and demand count, the accreditation agencies do not requirement such requirements. Bachelor is the basic requirement, Bachelor with four years working experience is equivalent to Master, Bachelor with eight years experience is equivalent to Phd in the Job market. However, if the job require education is a must, working experiences can not be substituted, such as professors.

If you can study for a doctorate, your salary will be much higher.

Which is better depends on what you want to do. Usually at least a bachelor (4 year) is required. After that graduate work includes Masters and Doctorates (some types of doctorates are Ph.D., M.D. (medical doctor), J.D. (juriprudence - judge), D.D.S. (dentist), and O.D. (doctor of osteopathy). A Masters is 2 years more than bachelor, and doctorate is usually about 2 more on top of that. Doctorates are best if you want to teach college level and some professions require them (medical doctor of course, dentist, etc.)

Undergraduate comes first, four years of that, after that you would become a graduate student. Graduate school includes medical programs to become a medical doctor, however sometimes that (like a legal education in the U.S.) is considered a professional degree whereas, say, a PhD (a doctorate) in literature is a graduate degree. I think postgraduate would refer to being, for example, a research fellow after graduate school.