Physically, they had a triangular face with a broad forehead, large compound eyes, a narrow pointed snout with a hooked jaw and sharp teeth, and long feathery antennae. They had barrel-like segmented torsos with four arms on the upper end. The lower pair was larger and the upper pair were almost vestigial, and they had three clawed hands. They also had two pairs of legs, the lower pair being thicker and used for movement, and the top pair being thinner and used to balance the body. (TNG novel: Imbalance)
Their exoskeletons ranged in different colors, from black to russet to copper, and they also varied in size as well. They had at least 3 known sexes: male, female, and neuter. When translating their language, their voices are multi-tonal, like many voices speaking at once, and their language consisted of hums, clicks, and buzzes. They had somewhat of a hive-mind, or a "group mind" that keeps track of each other to work in harmony. Each individual emits a sweet smelling "marker scent" determined by one's genetics and role in their society. Also when their names are broken down, it tells their place in society.
There were a number of Jaradan hives. One of the more well known hives was the hive Zel, which had colonized the planet Beltaxiyan Minor. These hives also contain a queen. It was on this colony that a madness disease affected their group mind, making a large number of them violently psychotic. This problem was solved by the crew of the Enterprise. (TNG novel: Imbalance)
The Jarada were reportedly xenophobic to the point of being "fussy" according to Commander Elias Vaughn. The Starfleet Captain who made first contact with the Jarada mispronounced a single word during his overtures, setting the process back twenty years.
The Jarada typically refused to allow aliens on their planet, having even shot down a vessel attempting to make an emergency landing. However, in 2376, Commander Vaughn was able to trade intelligence information on the Iconian gateway network in exchange for the Jarada allowing 500,000 Europani refugees to cross a doorway onto their planet. (DS9 - Gateways novel: Demons of Air and Darkness)
After Jarada warships inexplicably attacked the USS Defiant, all diplomatic relations between the Federation and the Jarada broke down, although neither the Federation Council or Starfleet Command were concerned. (DS9 - Mission Gamma novel: Twilight)