The law of conservation of energy states that all energy in the universe is conserved. It can not be created nor destroyed, but it can be interchanged between mechanical and non-mechanical energy.
The Law of Conservation of Energy is 0 = (d/dr + Del)(E + mcV) where E is the real or Potential Energy ( electrical, gravitational, etc) and mcV is the vector energy of the mass m and c is the speed...
In both cases, there is a quantity that doesn't change over time in a closed system - the conserved quantity. Both laws of conservation can help you solve certain problems that might be quite tricky...