Tricky conceptual problems that should not take more than a minute if you know how to solve it. Many of them can be done in your head, and the ones that require calculations only have a few steps. 100 problems total. Multiple choice. You get a real score that will help or hurt your admissions chances.Quals/Prelims:
More complicated problems that often require a lot of algebra, integrals, manipulations etc, even though they deal with very simplistic systems like those on the PGRE. (e.g. instead of a multiple choice question about how a pendulum would behave on the moon on the PGRE, you might have to solve the lagrangian eqs. of motion of a double pendulum on the quals). Doing well is often not as much about one's ability to think, but about having seen similar problems in one of the published qual prep books or on released past versions of the university's exams. There are often fewer than ~10 problems total on the test. The test is generally pass/fail and your exact score is not as important.
You can see examples of what the difference is by looking at old tests. 4 old PGRE's are avilable online. You can find qual problems in prep books like "A Guide to Physics Problems (Part 1 and 2)" or "Problems and Solutions on (Mechanics, EM, Quantum, Stat mech etc)". Also, many grad programs have old tests posted on their websites. Here is Princeton's