Grain structure refers to the arrangement and size of individual crystals, or grains, within a material, particularly metals and alloys. These grains form during the solidification process and can significantly influence the material's mechanical properties, such as strength, ductility, and toughness. The characteristics of the grain structure are determined by factors like temperature, cooling rate, and the presence of impurities. The grain boundaries, which are the interfaces between different grains, play a crucial role in how materials respond to stress and deformation. Smaller grains generally lead to stronger materials due to the increased number of boundaries that impede dislocation movement. Understanding grain structure is essential in fields like metallurgy and materials science, where it helps in designing materials for specific applications.