Cardassians is a hostile alien species found in Star Trek, they are especially prominent in the DeepSpace Nine series but dating back to the Next Generation - Cardassians were introduced so that the Federation could have an enemy species that was capable of being interacted with (unlike the Borg , who were without emotion or individuality).
Having evolved from cold blooded reptiles Cardassians retained many of the characteristics of the earlier species, such as a preference for darkness and warmer environments. They were noted for their intolerance to colder environments.
The Cardassians were once a peaceful people. Later known as the Hebetians they had a rich, spirtual life. However they were unable to cope with the changing climate of their world, which transformed the verdant Cardassia into a desert world. Millions of people died during this time, and a totalitarian dictatorship arose. The Cardassians decided expansion was the only way to survive.
Cardassians have a Unitarian, often xenophobiic view of the galaxy. Cardassians often beleive that they are the superior lifeform in the universe, and that it's their right (or even responsibility) to "guide" lesser lifeforms to a more enlightened state of being. In practice, this usually meant the enslavement of other species along with the strip mining of their planets for resources.
Cardassians also put a heavy focus on the idea of sacrifice, specifically the sacrifice of their own lives for the state. This is an idea prevalent through Cardassian pop culture and art, such as the novel "The Neverending Sacrifice" becoming lone of their most popular and respected works of literature.
As much as Cardassians are loyal to their union, they believe it to be infallible. Cardassian Trials always end in prosecution, as they believe that no innocent being would be accused in the first place.
Gul Dukat is perhaps one of the most well-known individual Cardassians in the series and is also one of the major antagonistic forces of Deep Space Nine (though he was also an ally at times).
Elim Garak is normally not villainous but his Mirror Universe counterpart is a vicious and sadistic Gul in the Cardassian army and one of that universe's many antagonists.
In the 2370s, the Cardassians agreed to become part of the Dominion. With their Dominion allies they fought a war against the Federation, Klingon Empire, the Romulan Star Empire, and other allied powers. The Dominion eventually began treating the Cardassians as a conquered race. When the Cardassian military had enough and turned against the Dominion. In retaliation the Female Changeling ordered her Jem'Hadar troops to commit genocide against the Cardassians. She later ordered her troops to stand down after linking with Odo, but by then Cardassia had been devastated, with over 800 million Cardassians dead.
Following the end of the war the Federation allies occupied Cardassia. They worked to stabilize Cardassia and provide relief to the surviving population. This included sending experts such as Keiko O'Brien to reestablish the planet's ecosystem. Seeing how previous governments had brought them to their current state many Cardassians worked to institute a new government that encouraged personal freedom and practiced Democracy.
Following the war the Caradassians maintained good relations with the allies, and When the Typhon Pact was formed by rivals to the Federation the Cardassians signed on to the Khitomer Accords, deepening the alliance between the Cardassians and their former enemies.