What's my job title?
You should list whatever your official job title is, as designated by the university.  (If you aren't sure, ask whoever hired you.)  You should do this even if the title is vague or misleading, in which case you should add a few comments on your actual job duties.

One reason to stick to the exact title is CV verification.  Some employers (both academic and non-academic) actively verify CVs, and if they call up your university and ask whether you were employed at job X during a certain span of time, then it's best if they get a definitive "yes" rather than a "kind of" or "there's no such job title".  In particular, some universities care about distinctions between seemingly similar titles, such as teaching assistant and teaching fellow, so you should make sure you aren't inadvertently awarding yourself a title that your university considers fancier or fundamentally different.