Everyone in the physics PhD program here has to TA at least one quarter. Currently, the department requires incoming students to take physics 500:
"This course, designed for graduate students, includes discussion of teaching, techniques and materials necessary to teach physics courses. One meeting per week with course instructors, one meeting per week in an assigned recitation section, problem session, or laboratory section. Students are required to take a total of two units of Physics 500."
As a TA, you might be assigned to be a lab TA, a lecture TA, an LTAC, or a grader. Roughly, here are the responsibilities of each:
Lab TA: run labs (~2h each x ~4sessions), grade labs
LTAC: guide TAs on lab, generally make sure labs are running smoothly
Lecture TA: run discussion (~1-2h each x ~2sessions), grade scantrons/tests
Grader: Grade