Transcranial Direct Current Stimulation (tDCS) is a type of neurostimulation where you apply a very small level of direct current through one electrode (Anode) and it flows through the brain to another electrode (Cathode).
This has a pronounced physiological reaction. The direct current creates a condition under the electrode that affects the neurons: the basement membrane of the neurons at the anode become more positive making them more likely to fire, and the basement membrane of the neurons at the cathode become more negative making them less likely to fire.
Through this simple method, tDCS has been researched to increase insight, improve learning of new things, and also help treat depression and stroke.
For the cognitive enhancement, it is necessary to be performing the task you want to learn while you are stimulating (or inhibiting) with a TDCS device the areas that will be neurologically active during the learned activity. This way the neurons are more likely to fire, and hence increasing brain plasticity.
But what is happening electrically?(For a more detailed discussion on the Electrical Nature of tDCS go to the blog)
The exact pathways of current from the anode electrode to the cathode electrode through the brain is not entirely known. The modeled circuit of the head (including skin, periosteum, skull, brain..) can be considered a simple design of just a resistor. But in reality that is not the case at all with so many non uniform materials the current must flow through.
It is more likely a much more complex system with some current flowing along the connective tissue in the scalp, and some flowing through the brain. It is generally known that the current intensities are highest under the anode and cathode electrodes; but from a bigger picture of current density to increased positive charges at the basement membrane is a large step (is it capacitive from the skull, with charge buildup on both sides?) I imagine we will soon know more.