In an electrical networks, there are closed loops and knots.

1. On a knot, all the currents I_n flowing "into" the knot will also have to flow out again (at the same moment).
2. The total voltage U of each closed loop is 0. You can choose any loop, as long as it is closed. For this to be possible, you have to assign directions (arbitrarily but consistently) to each of the elements' voltages.