What is the difference between a teaching assistant and an instructor?
No, there really is no universal definition of the terms; I have been a "teaching assistant" and an "instructor" at the same school for basically the same position.

The only thing that I would say is that the term "teaching assistant" tends to imply a position that does not have significant lecturing responsibilities, although she may be responsible for nearly everything else in the course (creating and grading homework and exam problems, interacting with students, conducting recitations sections, and so on). Note that this does not mean that the TA might not carry out a "spot lecture" or two; but this is not an expectation of the position overall. 

In general, I would avoid this problem by doing two things:


Listing on my CV the "job title" that the school assigns to the role you carried out.
Provide a short list of the duties your position entailed.


In this way, there is no ambiguity or misconception that can result, since you're providing all the information needed to understand the breadth of your teaching experience.