Graduate programs in most fields use the Graduate Record Exam (GRE) as one component of the admissions process. Expect to take the general GRE exam regardless of the type of graduate program that you will be applying to. This is a three hour exam that tests verbal, math, and writing skills (see http://www.ets.org/gre for more details).
Many Physics graduate programs also require the GRE Physics subject test, which specifically tests on knowledge of undergraduate-level physics.
The pages below offer study suggestions and preparation materials:
- GRE Prep Info: www.greprepinfo.com
- Magoosh: https://gre.magoosh.com/
- Masters Degree Net: https://www.mastersdegree.net/how-to-study-for-the-gre/