Brittle material breaks while little to no energy is absorbed when stressed. The material fractures with no plastic deformation. The material in the figure below marked with (a) shows what a brittle material will look like after pulling on a cylinder of that material. Typically, there will be a large audible snap sound when the brittle material breaks. A brittle material is also known as a material having low ductility. A stress-strain curve for brittle and ductile materials is shown in the figure below. We will talk more about ductile materials in the next section.
You may be asking: why are ceramics so much more brittle than metals? It has to do with the bonding. In metals, their metallic bonds allow the atoms to slide past each other easily. In ceramics, due to their ionic bonds, there is a resistance to the sliding. Since in ionic bonding every other atom is of opposite charge when a row of atoms attempts to slide past another row, positive atoms encounter positive atoms and negative atoms encounter negative atoms. This results in a huge electrodynamic repulsion which inhibits rows of ceramic atoms from sliding past other rows. In metals, the sliding of rows of atoms results in slip, which allows the metal to deform plastically instead of fracturing. Since in ceramics the rows cannot slide, the ceramic cannot plastically deform. Instead, it fractures, which makes it a brittle material.