First of all, it's important to understand what crystalline materials are and the concept of microstructure.
Familiar materials such as metals, minerals or ceramics are crystalline. In crystalline materials, the atoms that form the material are arranged to repeat periodically in space. The imaginary three-dimensional grid of points on which the atoms sit is called the crystal lattice. Of course, the size of the atoms and the distances between the repeating groups of atoms are tiny. For example, in aluminium, the atoms are arranged at the corners and face centres of a cube. The length of the edge of the cube is 0.405 nanometre - about 200,000 times smaller than a human hair. (1 nanometre is 10-9m).
At the size scale of atoms, the crystalline structure of material is very regular. Sometimes, atoms can form single crystals where the crystal structure is uniform over many millimetres. Everyone is familiar with the appearance of natural crystals of minerals such as quartz. In these cases the shape and symmetry of the crystal reflects the underlying regularity in the atomic structure. Single crystals can also be fabricated. For example, the single crystal silicon wafers used in microelectronics are up to 300 mm wide.
Left: The unit cell for aluminium. The atoms are at the corners and face centres of a cube.
Right: The unit cell for alumina containing aluminium atoms (red) and oxygen atoms (green).
More often however, the crystal structure is uniform over only short distances. Material is commonly formed of an aggregate of single crystal grains. Such material is called polycrystalline and the size of the grains can range from nanometres to being visible to the naked eye. Even within the single crystal grains the lattice is not perfect and can contain defects which have important effects on the behaviour of the material. The microstructure of a material refers to the assemblage of grains together with other microscopic constituents such as pores and inclusions.
Materials commonly used in engineering such as steel and aluminium are polycrystalline so it is important to have techniques that allow us to analyse their structure in detail. From the perspective of EBSD there are two important characteristics of polycrystalline materials. Firstly, the crystals in the different grains have different orientations. This means the edges of the crystal lattice point in different directions in different grains - if you can't imagine this it will be made clearer in the following sections! Secondly, polycrystalline materials contain regions where the different grains meet called grain boundaries.