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.

Eg.  Electricity, magnetism and light are different phenomena and have different laws of physics for each of them. These are unified under theory of electromagnetism;

Reduction: It is the effort to solve a complex problem by breaking it into simpler parts.

Eg. Temperature of a system is reduced to average kinetic.