Cytology is the study of individual cells and cytopathology is the study of individual cells in disease. Sampled fluid/ tissue from a patient is smeared onto a slide and stained (see techniques). This is then examined under the microscope by the anatomical pathologist to look at the number of cells on the slide, what types of cells they are, how they are grouped together and what the cell details are (shape, size, nucleus etc).
This information is useful in determining whether a disease is present and what is the likely diagnosis. Cytology is most often used as a screening tool to look for disease and to decide whether or not more tests need to be performed.