The Breen suit looks almost exactly like what Princess Leia wears upon trying to infiltrate Jaba the Hutt's palace.
Not only is their appearance mysterious, but their behaviour perplex others as well. The Romulan have a saying, "Never turn your back on a Breen". This is shown when a Breen sacrificed himself or herself willingly during an escape attempt of several others from a Jem'Hadar-guarded asteroid prison.
The Breen have established mining facilities consisting of Breen guards and kidnapped workers (from spaceships), such as Tora Ziyal, Gul Dukat's daughter.
The Breen suit is usually thought of as refrigeration suits, but some speculate that the Confederacy is comfortable environment (from the Cardassian point of view). The helmet of the suit consist of a glowing green visual piece.
Even though their language is entirely electronically coded and then broadcasted outside of the suit, they still can be translated with ease by the Dominion. The leading representative of the Confederacy to the Dominion is the Thot (similar in rank to a General).
Although the Breen appear almostly exclusively in episodes of Star Trek: Deep Space Nine, one Breen appeared as a hologram enhanced by the Hirogen in Star Trek: Voyager.