A rubric is a coherent set of criteria for students' work that includes descriptions of levels of performance quality on the criteria. Sounds simple enough, right? Unfortunately, this definition of rubric is rarely demonstrated in practice. The Internet, for example, offers many rubrics that do not, in fact, describe performance. I think I know why that might be and will explain that in Chapter 2, but for now let's start with the positive. It should be clear from the definition that rubrics have two major aspects: coherent sets of criteria and descriptions of levels of performance for these criteria.
The genius of rubrics is that they are descriptive and not evaluative. Of course, rubrics can be used to evaluate, but the operating principle is you match the performance to the description rather than "judge" it. Thus rubrics are as good or bad as the criteria selected and the descriptions of the levels of performance under each. Effective rubrics have appropriate criteria and well-written descriptions of performance.