A stress-strain curve is a plot that represents the relationship between the stress and strain for a material. A given amount of strain (increase in length of a specimen / original length) generates different amounts of stress in different materials.
A stress strain curve gives several pieces of information. The zones of the curve indicate various physical phenomena as show in the image below:
Yield strength, or yield stress is the limit of proportionality of the curve. It is the point at which the stress-strain response deviates from a liner relationship. Any further increase in the load will result in yielding, or the development of plastic strains in the material.
Ultimate Strength, or ultimate stress is the maximum stress that the material can sustain. A further increase in load will result in material softening, and a reduction in the load carrying ability. The stress will reduce with increasing strain, until the component breaks or fractures.
[1] https://commons.wikimedia.org/wiki/File:Stress_Strain_Ductile_Material.pdf
This page is part of The Encyclopedia of Stresses in Solids