One distinguishing characteristic was their gill-like structures that extended from the bottom of their ears down the ridge of their jaws. They also had a single small ridge on the top of their noses and an olive skin complexion.
When the USS Voyager crew made first contact with them around stardate 54274.7, the Kraylor were engaged in hostilities with another Delta Quadrant species, the Annari. The Annari had the upper hand at the time, having established – and maintained for three years – a blockade of the Kraylor homeworld. The Kraylor, however, were in the process of delivering a prototype cloaking device to their homeworld aboard one of their medical transports. This was contrary to the claim they made to Voyager of merely trying to deliver medical and food supplies. (VOY: "Nightingale")
The Annari had already learned how to detect cloaked vessels using a scanning pulse, which greatly reduced the usefulness of the Kraylor ship's cloak. Despite this, the delivery of the prototype was a success, thanks to the leadership and cleverness of Ensign Kim in outwitting the Annari patrols. (VOY: "Nightingale")
A Kraylor starship was among those trapped in the Void in 2377. They joined Captain Kathryn Janeway's proposed alliance, and a Kraylor delegate was aboard Voyager when Janeway was showing the ship to Bosaal. They ultimately left the alliance when Janeway expelled Bosaal for destroying a Kinjal frigate to obtain a polaron modulator. (VOY: "The Void")