Here, you may give seminars. These are often about solving some tasks (e.g., "compute the Fourier transform of a triangle"). Sometimes it is just consultation where students can ask stuff about the lecture. You may als be asked to conduct lab experiments with students. Sometimes your only teaching consists of supervising student research projects, Bachelor's theses or Master's theses.

The lectures are usually given by the professor (head of the research group). Sometimes a PhD gives a lecture when the professor is away. But yes, it depends on your employment contract. If your contract involves a lot of teaching you might even give a whole lecture - though this is rarely the case for employees without PhD.