Crystalline solids have a well-ordered, repeating arrangement of atoms or molecules, forming a distinct geometric structure. This regular pattern gives them unique properties, such as sharp melting points and specific optical characteristics. Common examples include salt, diamonds, and quartz. In contrast, amorphous solids lack this long-range order, resulting in a more random arrangement of particles. They do not have a definite melting point and can soften over a range of temperatures. Examples of amorphous solids include glass, rubber, and certain types of plastics.