Depending on what was originally agreed upon, the report may take on many forms. We will focus on the most elaborate one: the formal, written report.
Other kinds of reports may comprise a verbal briefing, an informal memo in the form of an email or a quick report in a status meeting. Other variants include not documenting the results, but instead updating the requirement documentation (in whatever form: wireframes or otherwise) or directly creating tasks for the development teams. This is very common when the UX researcher is not an external consultant, or the UX study is integrated into an iterative, agile environment.
A written report benefits from usability principles, just like any digital interface. It should be well-structured, with clear headings and making appropriate use of color and visual material. We have seen reports where charts were displayed using color, and green was used to denote failed tasks. Of course, interpretation of color depends on culture, but...