The structure of a crystal can be seen to be composed of a repeated element in three dimensions. This repeated element is known as the unit cell. It is the building block of the crystal structure. We define the unit cell in terms of the lattice (set of identical points). In three dimensions the unit cell is any parallelepiped whose vertices are lattice points, in two dimensions it is any parallelogram whose vertices are lattice points.
Of course this definition means that there are an infinite number of possible unit cells. So, in general, the unit cell is chosen such that it is the smallest unit cell that reflects the symmetry of the structure. There are two distinct types of unit cell: primitive and non-primitive. Primitive unit cells contain only one lattice point, which is made up from the lattice points at each of the corners. Non-primitive unit cells contain additional lattice points, either on a face of the unit cell or within the unit cell, and so have more than one lattice point per unit cell.
It is often the case that a primitive unit cell will not reflect the symmetry of the crystal structure. A suitable non-primitive unit cell will be picked in such cases.
It was mentioned above that the (eight) lattice points at the corners of the unit cell contribute only one lattice point to the cell. This is because the lattice points at the corners are shared between eight unit cells. Each corner lattice point therefore is equivalent to 1/8 of a lattice point per unit cell. Similarly lattice points on the edge of a unit cell are shared among four unit cells and are worth 1/4 of a lattice point per unit cell. Lattice points on the face of a unit cell are shared between two unit cells and are worth 1/2 of a lattice point per unit cell. Lattice points contained entirely within the unit cell are worth one lattice point per unit cell.
The most common types of unit cell are the primitive(P) unit cell with one lattice point per unit cell; the face centred(F) unit cell with additional lattice points at the centre of each face and four lattice points per unit cell; and the body centred(I) unit cell with a lattice point in the middle of the unit cell and two lattice points per unit cell. Other cell types are the C face centred unit cell and the rhombohedral unit cell.