Unification : it is the act of unifying the different laws valid for different phenomena in to a single theory that explains all the different phenomena.

Example – Newtons law of gravitation accounts for (a) fall of an apple to the ground (b) motion of satellites around planets

Though both are different fields of physics, the law of gravitation unifies them

Reduction : it is breaking down of a complex system in to simple constituent systems  such  that laws of physics can be applied on these systems and we can understand the working of the complex system.

Example – Thermodynamics is a branch of physics which deals with temperature, internal energy and entropy Later developments in the fields of physics like Statistical mechanics uses the concept of thermodynamics