Your duties as the TA will vary from course to course and from professor to professor. Some professors may expect more of you than others, so you should ask them to spell out your responsibilities in some detail before the course begins. If what the professors wants you to do sounds like more than 20 hours per week, then say so. Traditional duties include:
Occasionally, professors may ask you to lecture, cover new material in quiz sections, make up test questions and/or homework assignments, setup software, hold review sessions before exams, etc. You'll also have to answer lots of questions received in e-mail.
See the following sections for more information on specific duties.