A master's degree usually takes two years (or four semesters) to complete if the student is enrolled full time. Ph.Ds take at least five or six years, but can be more depending on the individual student.
Most master's degrees require completing of a thesis, which is a substantial research paper written under the supervision of professors. Completion of a Ph.D program requires a dissertation, which is extensive research, sometimes hundreds of pages long, on one specific topic.
A master's degree is considered a professional degree, which helps people prepare for careers in their chosen field. It is similar to a bachelor's degree, except that it is more specific, and provides higher qualifications. A Ph.D is more of an academic achievement, and most people who receive their Ph.Ds work as researchers of professors.
Ph.D and Master's degree programs are expensive, but since Ph.D programs are several years longer they are typically more expensive. However, there is usually more funding and financial aid available for Ph.D students. Some PhD programs even guarantee at least partial funding to all of their students.
While both types of degrees are highly competitive, much more so than undergraduate programs, acceptance into Ph.D programs is typically more competitive. Neither is easy to get into, however, and both require hard work and dedication.