These two terms describe the cell wall of bacteria. For bacteria, the cell wall is the barrier to the outside, like the peel of an orange, for example. The peel protects the inside of the orange and is quite stable. If you look closely, however, you will see that the peel is not perfectly sealed. It has small pores through which molecules like air can pass. This is why it can happen that the orange dries out and no longer tastes as good. The cell walls of bacteria have a similar function. Bacteria are divided into thick and thin cell walls. "Gram-positives" have thick cell walls, and "Gram-negatives" have thin cell walls, but they have a further protective layer over the cell wall. The different structure of the cell walls is important for the classification of bacteria, because many of their characteristics can be traced back to this. In order to determine the thickness of the cell wall, the bacteria can be stained dark violet and then decoloured using a special staining technique. Gram-positives appear dark violet because the colour gets stuck in the thick cell wall. Gram negatives, on the other hand, cannot fix the color during staining because of their thin cell wall and turn pinkish red. More details about this staining technique can be found under "Methods". In contrast to prokaryotes, most eukaryotes have no cell wall, with the exception of plants. Cell walls give stability to plants, but this prevents them from moving, like a tree for example.
