Direct current (DC) is the continuous flow of electricity through a conductor such as a wire from high to low potential. In direct current, the electric charges flow always in the same direction, which distinguishes it from alternating current (AC).
Direct current was used originally for electric power transmission after the discovery by Thomas Edison of the generation of electricity in the late nineteenth century. It has mostly been abandoned for this purpose in favor of alternating current which is much more suited to transmission over long distances. DC power transmission is still used to link AC power networks with different frequencies.
DC is commonly found in many low-voltage applications, especially where these are powered by batteries, which can only produce DC. Most automotive applications use DC although the generator is an AC device which uses a rectifier to produce DC. Most electronic circuits require a DC power supply.
Although DC stands for 'Direct Current', DC is generically used to refer to constant polarity voltages. Some forms of DC vary wildly in voltage, such as the raw output of a rectifier. Running them through an RC low-pass filter will produce more stable voltage.
Direct current installations usually have different types of sockets, switches, and fixtures, mostly due to the very low voltages used, from those suitable for alternating current. It is usually extremely important with a direct current appliance to not reverse polarity unless the device has a bridge to correct for this. (Most battery-powered devices don't.)