The PhD programs in the US are quite rigorous. Research work in the US generally lasts for about 3 to 8 years (some finish it more quickly and some take longer). The time taken to complete different courses varies because of the differing nature of research between the courses, and because of varying difficulty of coursework, languages and length of thesis.
In the US, quite a few departments (mostly Science) conduct a comprehensive examination in the first year of doctoral study. This exam tests a student's knowledge on a particular subject, from his past courses (bachelors and masters).
Most doctoral students start with their master's thesis by the start of second year. Master's thesis either consists of research culminating in a paper, presentation, and defense of their research.
However, most programmes do not have a Master's thesis. These programmes primarily focus on workshops, assignments, practical and other such real-world experience.
The second and the third year of doctoral studies require candidates to appear for exams. Some of these exams include Qualifying Exams (also called the Quals), General Exams and even PhD Candidacy Exams (also called the Candidacy). These exams test a candidate's grasp over the broad field of his choice and, also, Special Field Examination (also known as Specials), which test students in the subjects that they are primarily focusing on. These exams may be oral or written. In case the candidate has not studied the subject previously, the exam will be based on specific preparatory reading.
The doctoral candidates, then, primarily focus on their research.
Many departments, especially those, in which students have research responsibilities, offer fee-waivers and stipends that pay for most expenses. At some universities, there may be fixed stipend established for all PhD students. The terms vary quite a bit, and may contain fellowships or scholarships. Due to promotion of research related activities and pressure from students, the stipends have been increasing in most universities.