In the ANOVA models can contain fixed and/or random factors. For a fixed (effect) factors, we are interested in studying the specific levels in that factor. For a random (effect) factor data is collected for a random sample of possible levels, with the hope that these levels are representative of all levels in that factor. This approach can be appropriate where there are a large number of possible levels.
Models that contain both fixed and random factors are called mixed models. We will also consider nested models where one factor is lower in the hierarchy compared to another factor.