Unit cell can be considered as the building block of a crystal. It has the same symmetry as the entire crystal. When we arrange the unit cell in 3D, we get the bulk crystal. In other words, it can be described as the smallest volume which when repeated in all direction gives the crystal. The three edges a, b, c along the axis and angle between them α, β and γ is termed as lattice parameters. In 3D it is better to consider a parallelepiped as a unit cell.
Unit cell can be of primitive as well as non-primitive type. A primitive cell is a minimum volume unit cell and has only one lattice point in it and the latter contains more than one.
In the given figure below, a simple cube is a primitive cell. No.of atoms per unit cell are one for it. The rest two is non-primitive. No. of atoms per unit cell are 2 and 4 respectively.
Simple cubic (P) Body-centered cubic (I) Face-centered cubic (F)