Cardassian's are a militaristic race easily recognized by their grey skin and neck ridges, their children are taught at a very early age to be ruthless and to put the welfare of the state before anything else, they undergo rigorous mental training in order to be immune or heavily resistant to interrogation and even telepathy, they also all have a photographic memory.
Before the military took over Cardassia was a beautiful place renowned for their arts and excellent record keeping, but due to an increasing lack of resources the Central Command was put in charge and Cardassia has since been a police state. But starvation still runs rampant - and excessive laws are common such as requiring all children to donate one of their molars at the age of 10 to the bureau of identification or every criminal has his sentence decided before the trial has begun, the trial being more of a "example" to the general public.
Cardassian's are also well-known for their exceptional intelligence agency, the Obsidian Order, the order members out rank all but the highest ranks of the military. Most Cardassian citizens live in fear of the order due to their renowned cruelty and their heavy surveillance, they have an uncanny ability to weed out traitors and know the smallest details of the personal lives of anyone they happen to find interesting.
The Cardassian's were created by the writers of the Star Trek: The Next Generation episode called "The Wounded".