What is the difference between a yield strength and tensile strength?

Tensile strength is a measurement of the maximum deformation a certain material can take without necking

Yield strength can be  defined as the amount of stress where a predetermined amount of plastic (irreversible) deform. occurs 

Tensile strength is a measurement of the maximum deformation a certain material can take without necking, not necessarily its point of failure.  

Yield strength can be  defined as the amount of stress where a predetermined amount of plastic (irreversible) deformation occurs. If the applied stress is lower than the yield strength, the deformation is always elastic,( reversible ) . 

Yield strength is the capacity of a material, at which it starts to deform plastically - i.e. the material would not retain its original shape, if stressed beyond the yield strength, once the stress is removed.

The tensile strength is the maximum tensile stress capacity of a material till its breaking point.