One of the main decisions you will need to make is whether to pursue a master's degree or a Ph.D. For some this decision is easier than for others: some professions, such as college professor or research scientist, require a Ph.D. In other fields a master's degree is enough to qualify you for a better position and higher salary.
What is the difference?
Master's programs can usually be completed in anywhere between one and three years. Most programs have a set curriculum, require certain tests and lead to a Master of Science (M.S.), Master of Arts (M.A.) degree or other professional degree. Depending on the program, they may also require a thesis.
A Ph.D., which stands for Doctor of Philosophy, requires a great deal more time and effort to complete. Ph.D. programs can vary in length from three to eight years, most averaging about five. The focus of a Ph.D. program is usually on original research and the topic is generally left up to the individual. The culmination of the program is the dissertation, which is a publication with the findings of the student's original research. Many Ph.D. programs also require some form of comprehensive examination.
What's right for me?
This really depends on the nature of what you are studying and what you want to do with your graduate degree. If your interests lie in changing the direction of your career or increasing your earning potential, you should probably be considering a master's program. Because these programs are less time consuming, you could actually complete a master's on a part-time basis while keeping your current job.
A master's also brings more immediate results than a Ph.D. If your goal is to boost your value in the job market, this is the way to go.
If, however, you have a passion for the naked mole rat or Elizabethan footwear - and you wish to center your career, research, and basically your life around a certain topic - then the Ph.D. is for you. Dedicating years of your life to a research project is a noble endeavor, but it can be difficult on many levels. If you are considering a Ph.D., you should be fairly sure of your dedication and desire to learn and conduct research on your chosen topic.