TAs are Teaching Assistants. They are usually graduate students working as faculty assistants to supplement their graduate expenses. Sometimes advanced undergraduates become TAs when their work in a particular area is outstanding and they have gained the confidence of senior faculty.
 
Sometimes the very mention of TAs can have a negative connotation when it comes to teaching. That's because at many large universities, especially in introductory courses, TAs sometimes do a significant amount of the teaching. Even if they are not involved directly with the instruction of the class, teaching assistants can be a large part of lab activities and counseling for the students.
 
The reason for the negative perception has nothing to do with the quality of the TA's teaching. Some TAs can be genuinely exciting and motivational. The problem comes from students and parents who feel that for the high price of tuition, faculty should be doing the teaching, not a graduate student who is also a teaching assistant.
 
In large introductory courses, the teaching assistant can also handle the administration of exams and grading. Many first-year students may never know that a graduate student is doing the teaching. The way to tell is to get a listing of courses and then check to see who will be doing the teaching. If the name of the instructor or professor is not on that course's departmental faculty listing, you may want to do some research. If you find out that you're dealing with a TA, you might be able to pick the course up later when a member of the faculty will teach it.
 
There is nothing wrong with Teaching Assistants. They serve a positive and productive function. If I had a choice, though, I'd prefer full-time faculty.