TA talks to others about students, what to do? I am an undergraduate student at a university in the US.

Unfortunately, I have a concern regarding one TA of a class I took this past year. This individual insults other students in the university during his TA session, and I also found out from other people that this TA has been talking to other people (not just the professor and TAs for the class) about people's grades in the class.

My question is, is it within my power to lodge a formal yet anonymous complaint that will be taken seriously about this?

I am concerned about my identity becoming known because this TA is a strong student, and seems to have a lot of power in the department (and is close with the professor teaching the class).
When you have a complaint about a teaching assistant for a course, you should generally address it to the instructor of record first. Even if you believe the professor will dismiss your complaint, going straight over the professor's head will make you look bad.

Here are some suggestions for making a well-received complaint:


Make your complaint in writing (email).
Do not include any subjective judgment of the TA's behavior, only objective observations of what you have seen and experienced.
Give specific evidence to support your claims.
Phrase it as if you believe the professor is not aware of this behavior, whether you truly believe this or not.
Don't offer suggestions on what actions to take ("I think you should prevent this TA from working in future courses"), just describe what you have seen.
You can specify what steps you would like the professor to take to keep your identity a secret from the TA when discussing the complaint with him. (Only you or someone close to your situation can know how identifiable such a complaint will make you, and what details need to be omitted to anonymize you effectively.) 


If the professor does not seem to take the matter seriously, you can escalate your complaint:


To the department head
To a student ombudsperson at your institution