The Sith Order was a sect of dark side Force users bent on destroying the Jedi.
They were characterized by their single-minded lust for power and the
brutal methods they were willing to use in order to acquire it. The
sect existed in many diverse forms throughout galactic history.
Originally, the Sith were a humanoid species that interbred with a group of exiled Dark Jedi,
but after the rise and fall of that people, the meaning of the word
'Sith' changed. In time, the Sith would not be identified by their
race, but by their dedication to the ancient Sith philosophy. There
were many factions of the Sith, the most notable being the Sith Empire and the Order of the Sith Lords.

Forged by fire
"The
Sith and the Jedi are similar in almost every way, including their
quest for greater power. The only difference between the two is that
the Sith are not afraid of the dark side of the Force. That is why they
are more powerful."
"The Sith rely on their passion for their strength. They think inwards only about themselves."
―Palpatine and Anakin Skywalker

"Without strife, your victory has no meaning. Without strife, you do not advance. Without strife, there is only stagnation."
―Yuthura Ban

"It is natural for him to
want to destroy me. It is not crude mundane ambition, as it would be in
an ordinary man; it is part of his growth. And of course it does not
offend me—it is why I chose him. But he needs to grow still further."
―Emperor Palpatine, on his apprentice, Darth Vader

Sith philosophy emphasized conflict. The Sith believed that it
empowered both individuals and civilizations by forcing them to change,
grow, and evolve. Conflict ensured that only the strong would survive,
thus leading the Sith down the path to perfection. Sentients only
deserved what they were strong enough to take and those too weak to
protect themselves were fair game.

The pacifistic teachings of the Jedi were viewed as
obstructions to progress; lack of conflict led only to stagnation and
decline. Morality was treated as an obstacle to be overcome. It got in
the way of a Sith's ability to recognize and grasp opportunities for
advancement.

Empowered by passion
"What
fuels your power with the Force but your passion? The stronger, darker
emotions. Anger, hatred, fear… these passions empower us."
―Yuthura Ban

The core of a Sith's power came from emotion. The Jedi taught that
fear and anger were negative emotions that must be overcome, but the
Sith understood that every living creature was subject to such
passions. They believed in harnessing rather than suppressing these
natural urges. The Sith did not endeavor to place themselves beyond
passions as the Jedi did – on the contrary, they believed that emotion
was required in order to attain a holistic understanding of the Force.

However, not all emotions were embraced by the Sith. They,
like the Jedi, considered 'love' a dangerous emotion, but for different
reasons. The Jedi discouraged love because it led to attachment, but
the Sith taught that love led to mercy, which was anathema to them.

Many Sith Lords
adorn themselves with Sith tattoos, as a symbol of their devotion to
the order. The tattoos often have an alternating pattern of red and
black, with many runic symbols included. Sith tattoos are incredibly
painful to have applied, as they often cover the entire body. Examples
of tattooed Sith Lords include the infamous Darth Maul, Darth Talon, Darth Wyyrlok, Darth Ruyn, and many other members of Darth Krayt's New Sith Order.

The "True Sith" was an obscure and largely unknown order of adherents to the dark side of the Force. Darth Traya told the Jedi Exile
that the order had been in existence for tens of thousands of years and
were silently waiting in the Unknown Regions during the time of the Jedi Civil War.

According to Darth Traya, the "True Sith" had existed for millennia prior to the rise of the ancient Sith Empire. She and Revan believed that it was they who had goaded the Mandalorians into attacking the Galactic Republic, starting the Mandalorian Wars. Kreia stated that both Malachor V and the graveyard planet, Korriban,
had once been planets at the edges of that mysterious Empire, and that
while these "True Sith" had forgotten those ancient worlds of the dark
side, they would remember.

The Sith Empire was a rich and powerful empire that was established by Dark Jedi banished from the Galactic Republic after their defeat at the Battle of Corbos during the Hundred-Year Darkness. After developing in complete isolation for thousands of years, the Sith eventually rediscovered the Republic. The Dark Lord of the Sith Naga Sadow led the Empire in an invasion during the Great Hyperspace War, but he was defeated. The Sith Empire was believed to have been utterly destroyed by Republic forces in the aftermath of the Second Battle of Korriban.

The Sith Empire was temporarily resurrected by the fallen Jedi Revan and Malak. Built upon a hardened core of veterans from the Mandalorian Wars and a host of Jedi converts, the fledgling new galactic power very nearly succeeded in conquering the Republic during the Second Sith War.
After the redemption of Darth Revan and the death of Darth Malak, their
Empire splintered into factions and eventually collapsed.

Order of the Sith Lords

The Sith Order under the last surviving Sith Lord, Darth Bane, was created as a result of the devastating Seventh Battle of Ruusan,
which saw the annihilation of the previous Sith Order. Bane's Sith
Order, known formally as the Order of the Sith Lords, was founded upon
the principles of Darth Revan's Holocron, wherefrom he formed the Rule of Two,
utilizing stealth tactics and treachery to achieve their means. It
would be this Sith Order, and its principles, that would finally
triumph over the Jedi and the Galactic Republic nearly 1,000 years
later. The Rule of Two would allow his eventual successor, Darth Sidious, to finally triumph over the Jedi and the Republic 1,000 years later.

Disciples of Ragnos

The Disciples of Ragnos was a Sith Cult under the leadership of Tavion Axmis,
who sought to resurrect the spirit of Marka Ragnos through ancient Sith
sorcery and take control of the galaxy. The cult was closely related to
the Empire Reborn movement of which Tavion had herself been part with the Dark Jedi Desann. The cult was ultimately defeated by Jaden Korr and the New Jedi Order.

Lumiya's Sith

Lumiya's Sith were a loosely collected group of individuals associated with Lumiya, self-proclaimed Dark Lady of the Sith
after the final death of Darth Sidious.Even with Palpatine's final
death, the Sith continued to survive. Their next Sith Lord was an Emperor's Hand named Lumiya. Darth Vader
had passed on to Lumiya some Sith teaching secretly from his master,
and the woman adopted the title "Dark Lady of the Sith" upon her
masters' death at Endor. She trained two apprentices: Flint, who was redeemed by Luke Skywalker, and Carnor Jax, who was killed by Palpatine loyalist Kir Kanos.

Soon afterwards she succeeded in convincing Jedi Knight Jacen Solo to join the ranks of the Sith.

New Sith Order

By the year 130 ABY, a new Sith Order had risen to take the place of Darth Bane's order. It is known that a Sith known as Darth Krayt began the order on Korriban, the ancient Sith burial world, around
30 ABY. In this new Sith cult, many of the followers sported a red-and-black tattooed appearance and used lightsabers
with an yorik coral-styled hilt design. This Sith Order was considered
heretical by the Sith of the past. Krayt spent over a century growing
and strengthening the new order, planning to unify the galaxy once more
under Sith rule. He and his new order made their presence known around 127-130 ABY by initiating the Sith-Imperial War, and then launching an attack on the Jedi on Ossus.