A Lab Report is where you write exactly what occurred in the experiment, under exactly what conditions, and clearly discuss the results and their relation to your hypothesis.
Be clear and specific. You are not only communicating your results to your professor, but you are also sharing your experiment with the scientific community to avoid future repetition and redundancy. To that end, it is important that you provide all of the necessary information in the order and format that is considered standard in the scientific community.
The Anatomy of a Lab Report