Direct current is, much like the name suggests, the flow of electrons towards a single direction. It is very simple to visualize direct current using the "water circuit" model; simply think of water flowing towards one direction inside a pipe. Common devices producing direct current are solar cells, batteries and dynamo generators. Almost everything can be developed to be powered by a DC current source and it is almost exclusively used in any low voltage, mobile and electronics applications.
DC current is pretty straightforward and almost everything is based on Ohm's Law (V = I * R), while the power of a DC load is measured in Watts and equals P = V * I (Watts).
DC Current / Voltage waveform (Hypothesis : 12V, 5A, 2.4Ohm load)
Because of the simple equations and its behavior, DC current is relatively easy to comprehend. The first electric power transmission systems, developed by Thomas Edison back in the 19th century, were using direct current. The long range distribution of DC however is problematic and it was soon replaced a few decades later by the significantly advantageous (at the time) AC current developed by Nicola Tesla.
Even though the commercial power grids of the entire planet nowadays are using AC, technology advancements ironically made high voltage DC transmissions more efficient over very long distances and/or extreme loads, such as when interconnecting separate AC systems like entire countries or even continents. However, AC continues to be the choice for low voltage commercial grids, for reasons explained later in this article.
Although AC is far easier to generate by using kinetic energy through a generator, batteries can only produce DC and this is why DC dominates low voltage and electronics applications. Batteries can be charged only by DC as well, which is why all AC power is instantly transformed to DC when a battery is a main part of a system. A very common example of this would be any automotive application, such as motorcycles, cars and trucks. The alternator (also known as dynamo) present in vehicles generates AC current which is instantly transformed to DC through a device called "rectifier", because a battery is present and most electronics need DC voltage to operate. Solar cells and fuel cells can also only produce DC, which can then be transformed to AC if necessary through a device called "inverter".