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).
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 will usually mean the enslavement of the enslavement of their planets.
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.