KCL and KVL

KCL (Kirchhoff's Current Law) KVL (Kirchhoff's Voltage Law)

This fundamental law results from the conservation of charge. It applies to a junction or node in a circuit -- a point in the circuit where charge has several possible paths to travel.  

KVL - is one of two fundamental laws in electrical engineering, the other being Kirchhoff's Current Law (KCL).

KVL is a fundamental law, as fundamental as Conservation of Energy in mechanics, for example, because KVL is really conservation of electrical energy.

KVL and KCL are the starting point for analysis of any circuit.
KCL and KVL always hold and are usually the most useful piece of information you will have about a circuit after the circuit itself.

Here's a simple circuit.  It has three components - a battery and two other components.  Each of the three components will have a current going through it and a voltage across it.  Here we want to focus on the voltage across each element, and how those three voltages are related.