From DT Online
As material samples are stretched, initially, the Stress is directly proportional to the Strain - producing the straight line as shown on the graph. The material is behaving in much the same way as a spring would under tension and, if the force is removed, the material returns to its original length. The material is obeying Hooke's Law and this phase is known as the Elastic Stage.
But if a force is continued to be applied beyond this phase, the material reaches a point at which it stays permanently stretched. This is the Yield Point or Elastic Limit and beyond this, during its Plastic Stage, the material will continue to remain permanently deformed until eventually it breaks.
Plastic deformation occurs when the applying force is sufficient to break the Inter-atomic Bonds, causing them to be rearranged as Dislocations move through the Crystal Lattice and is therefore not reversed when the force is removed.