The Sith have access to powers that the Jedi deem to dangerous to use, such as Force Lightning, Force Choke and Dark Side Corruption. Each Sith Lord is masterful in different areas of both Light and Dark powers, for example, Darth Sidious uses Lighting as his primary form of attack and can see into the future, Darth Vader is proficient in Force Choke and Lightsaber Combat, and Darth Nihilus can literally devour living souls.
Despite their reputation for being evildoers, not all Sith are inherently evil. They are driven by fundamentally selfish motives, unlike the Jedi who work for the benefit of everyone, and because of this many of them have led conquests of power and conquest, giving the Sith a bad reputation. Despite this, there are still some Sith who abstain from evil deeds, but in recent years they are few in number.The Sith were once a mighty Empire, consisting of many worlds in the Force Galaxy. Despite being under the leadership of Sith, these worlds prospered and this prosperity only came to an end during the reign of Darth Malak and the Rise of Trask Ulgo. Following this, the Sith Triumvirate formed, which was more concerned with destruction than leading an Empire.
Following this, and the introduction of the numerically limiting 'Rule of Two', the Sith were reduced to little more than a cult, and they would not achieve Galactic Dominance until the Galactic Empire rose thousands of years later under Darth Sidious and Darth Vader, and they controlled the events of the Clone War and the Sith-Covenant War. This brief reign ended when the last remaining Jedi, Luke Skywalker, destroyed the two Sith at the conclusion of the Galactic Civil War. The Sith did not rise again as a political body for several hundred years, after which they had returned to their roots as a more benign Galactic power.