In most cases the binding of a stain to the tissue may be described as an ionic bond. Stains are usually neutral salts and are classified as basic stains if the "colour" is found in the positive part (cation) of the salt. The "colour" of acidic stains are found in the negative part (anion) of the salt.
Basic stains react with anionic tissue components eg nucleic acids (DNA & RNA). Such components are described as basophilic (loves basic dyes).
Acidic stains react with cationic tissue components. Such components are called acidophilic or eosinophilic.
Slide 21 that you are looking at is a frog blood smear stained with haematoxylin and eosin (H & E). The cells are red blood cells.
Haematoxylin is a basic stain and stains the nucleus purple-blue, while eosin is acidic and stains the cytoplasm pink.
The cells are the pink structures, while the nuclei are the purple "dots" inside the pink cells.
It is important to understand and know the terms and principles, as these will be used throughout the course.
The affinity of a tissue component for a stain is controlled by its chemical characteristics. Not all stains will stain all components, while those that do react, may not all stain with the same intensity. For example, both collagen and muscle fibres are acidophilic and stain pink, but with different intensities. The tissue is thus differentiated and you will soon, with a little practice, learn to distinguish between the different tissue components.