Before understand dislocation let’s define defect. If there is any deviation from regularly arranged lattice, that is called defect. Let’s start first with fracture of bone. what happened when the bone fracture? Continuesly connected bone breake and dislocate. That means regular arrangement of bone is disturbed and creates stress field which we feel as pain . similarly, regularly arranged atoms are disturbed and create local stress field. We are visualising that stress field in one dimension. That is called dislocation. So dislocation is a defect. Basically, this dislocation classified into three categories.
1) Edge dislocation
2) screw dislocation
3) Mixed dislocation