What is exactly meant by "research experience" in grad application?
I would say all of those count as "research experience".  Which ones will be most valued depends on the nature of the program you're applying to, and the exact nature of the work you did.  In general, though, the more independent your work was, the better, and the more generic it was, the worse.  So if you were a research assistant but all you did was photocopy documents, that will not earn you many points.  If you were a research assistant who, e.g., conducted experiments, did fieldwork, participated in lab meetings, whatever, those will be worth more.  If you actually contributed to the writing of papers or presentation at conferences, even better.

As a rule of thumb, the more that what you did was something other people could not have done (i.e., it required your special expertise, not just "more hands'), the "better" it is.