The dark side of the Force is the darker aspect of the Force. Sith and Dark Jedi drew their powers from the dark side.
Some Jedi would sometimes draw from the Dark Side of the Force whenever they felt their loved ones were in danger. Ezra Bridger, for example, drew from the Dark side because he was driven to protect Kanan Jarrus from The Grand Inquisitor.
It was called The Bogan by Lasats and The Bendu.
Channeling intense and aggressive feelings, such as pain, fear, anger and passion would increase one's connection to the Dark Side. While its power would enhance the strength of their powers beyond their normal limits, it would also corrupt their personality and morality, usually creating an unstoppable drive for more power. The Sith, practitioners of the Dark Side, wielded their power to gain power and dominance over the galaxy.