Think of the PSAT as a baby brother of the SAT®. It serves as a practice test to show you what the SAT® is like. For most students, the PSAT serves as a good wake up call to start thinking about college application process. The PSAT is administered in October. While you can take the PSAT as a Sophomore if it is offered at your school, you should definitely take it as a Junior as your PSAT score could qualify you to enter National Merit Scholarship programs. The only real difference between the PSAT and SAT® is that the PSAT does not have an essay section.
Similar to the PSAT, the PLAN is a practice test for the ACT. It both shows you what the ACT will be like, and provides an indication of how you will perform on the actual ACT.