PBT stands for “paper based test,” the old pencil-and-paper format, usually proctored in large rooms. CBT stands for “computer based test,” usually offered in computer labs. Most exams are now available only as CBT, except for the GRE, which is still PBT but plans to convert to CBT only by 2008.
For most students, the biggest difference between PBT and CBT is the writing section: students who can touch-type have a distinct advantage in CBT tests. In multiple choice sections, the main advantage seems to be familiarity and individual preference, although students who “mouse” with their off hand (right-handers who “mouse” left, or left-handers who “mouse” right) have a small advantage because they can take notes or work problems with one hand while clicking or scrolling with the other.
Some CBT exams are sequential – you must answer all questions in order; others are open – you can skip around, answering questions in any order. Also, some CBT exams are adjusted – when you answer a question incorrectly, the exam skips similar, more difficult questions and sends you to a different track of questions; others are set – everyone answers the same set of questions. Most students are more comfortable with open, set exams, but students cannot choose their own format. Be sure your practice exams use the same format as the exam you will be taking.