Kirchhoff's Voltage Law (KVL), which is also known as Kirchhoff's Second Law, states that the sum of all voltage sources in a circuit loop equals the sum of all voltage drops in the same loop.  This law is also sometimes stated as follows: the net electromotive force (e.m.f.) within a circuit loop equals the sum of all potential differences around the loop.  In this statement, e.m.f. refers to a source of electrical energy while potential differences are drains of electrical energy.

Figure 1.  Kirchhoff's Voltage Law