(covers information from several alternate timelines)
It consisted of a small central core containing the drive system and three extended wing-like sections extending forward, mounted at 120 degree angles along the fore-aft axis. The ships were heavily armed with particle cannons (green or blue in color) and torpedoes. They were also highly decentralized in layout, possessing no central control room, reflecting the non-hierarchical nature of Insectoid culture. There was at least one room that had a viewscreen for communications. (ENT: "Countdown")
In early 2154, Enterprise NX-01 discovered the wreckage of a crashed Insectoid ship while traveling to the Azati Prime system. The Enterprise crew discovered that the adult crew had died when they transferred power from life support to power the on-board hatchery, protecting their young. (ENT: "Hatchery")
Two Insectoid ships participated in the Battle of Azati Prime, and others defended the Xindi weapon alongside Xindi-Reptilian warships in February of 2154. (ENT: "Azati Prime", "The Council", "Countdown")
In an alternate timeline in which the weapon was successful in destroying Earth, three ships of this type were part of a Xindi assault fleet sent to destroy the last remnants of Humanity in the Ceti Alpha system in 2165. During the battle, one was destroyed by fire from Enterprise and another was disabled by the Intrepid and another starship. (ENT: "Twilight")
Despite being introduced in the episode "Twilight", the final draft script for that episode gives no physical description of this type of craft.
The brief for the Xindi-Insectoid starship was that it should look very alien and be physically unlike anything ever seen before. (Star Trek: The Official Starships Collection issue 24, p. 11)
The exterior of the "Xindi-Insectoid starship" was designed by John Eaves, who hit upon the design idea after he had a lobster dinner.  Eaves was visiting the local market when inspiration struck. "I was looking into the ice chest and saw these king-crab legs and I thought, 'Wow, that would make a really creepy appendage to a spaceship,' so that was where those jaggedy crab-like jointed tendrils came from." (Star Trek: The Official Starships Collection issue 24, p. 12) Continued Eaves, "I got the assignment the next day to start working on the Xindi Insectizoid ship. Having those big buttery crab legs still on my mind, my subconscious mind doodled out the starship version of my dinner!!!!" 
As well as the crab legs, other sources of inspiration came from insect anatomy, such as cricket legs. "I was looking for anything that would give it a bug look," John Eaves clarified, regarding the Insectoid warship. "On the backs of crickets they have that serrated shell, and that's where I got the breakdown of the detail in the central body area. It's got that kind of faceted, louvered look, which came from the back of a cricket. Then I figured it should have some kind of eye port – that would be where the bridge or the command centre would be. So, for that I mimicked bug eyes, but not directly." (Star Trek: The Official Starships Collection issue 24, p. 12)
John Eaves proceeded by illustrating an initial concept drawing of a thin ship with long 'arms' that had three joints. Before submitting the initial design to the producers, Eaves produced a comparison drawing in which the Insectoid starship was beside Enterprise (on its right) and (on its left) the spherical Xindi weapon which attacks Earth in "The Expanse". (Star Trek: The Official Starships Collection issue 24, p. 12) "I was very happy with the first pass of this flying seafood dinner starship," enthused Eaves, "and so was my boss, Herman Z. We drew up a couple of size charts and off it went to Mr. Berman's office."  Rick Berman was enthusiastic about the basic concept of the design but requested one major change, specifically wanting to shorten the extremely long arms. (Star Trek: The Official Starships Collection issue 24, p. 12) Explained Eaves, "The drawing came back with a hard line through the middle of the legs that translated out to be, 'cut em off here.'" 
Returning to the drawing board, John Eaves eliminated the third limb of each of the Insectoid ship's arms, resulting in the design still having a "knuckle," though pulled back to a smaller point. Eaves featured the shortened arms in his second and final pass. To his delight, this revised drawing was given the go-ahead, with unusually few drawings having been produced before acceptance. (Star Trek: The Official Starships Collection issue 24, pp. 12 & 13) "[It] got the approval in red ink and an actual written note!!!" Eaves exclaimed. "WOW, that didn't happen too often!!!" The design was approved on 2 October 2003. Eaves rendered a color illustration of the finalized design. It was deemed important, about the same time, that the coloration of the craft be slightly different from that of the Xindi-Reptilian warships.  The final design of the Insectoid vessel was approved by not only Rick Berman but also Executive Producer Brannon Braga, before the design was sent to Eden FX. (Star Trek: The Official Starships Collection issue 24, p. 11)
At Eden FX, Pierre Drolet built and rendered the design as a CGI model. During the build, Drolet had to make some adjustments, as he was mistaken about the flight direction of the ship. Eaves was pleased about Drolet's input, feeling the arrangement of the ship's central hub was refined and improved by Drolet. "I love how he split the central core of the ship into two segments," Eaves remarked, "instead of a symmetrical three piece section to match the three extrusions."  Eaves was proud of the design in general, feeling it genuinely offered something different to the kind of ships normally presented on Star Trek. (Star Trek: The Official Starships Collection issue 24, p. 12)
Although John Eaves had designed the Insectoid starship to fly with two arms pointing up and one pointing down, the visual effects team, when the time came to create shots of the vehicle, decided the ship would look cool if it spun around. So, the warship ultimately appeared on screen in virtually every possible orientation. (Star Trek: The Official Starships Collection issue 24, p. 18)
Eaves briefly returned to his design, two months later, to produce a crash scene design – as reference for the modelers at Eden – for the episode "Hatchery". ; Star Trek: The Official Starships Collection issue 24, p. 13) In the final draft script of that outing, the ship was initially described as "a crashed ALIEN VESSEL that's plowed a trench across the rocky surface [of a planet] (NOTE: We'll eventually learn this is a Xindi-Insectoid ship.)" The same scripted scene referred to the craft as "exotic" and the script went on to say (in a subsequent scene), "The ship's sustained heavy damage... several breaches are visible in the outer hull."
Star Trek: Enterprise's art department designed the interiors of Xindi-Insectoid vessels by taking a cue from the fact the Insectoids themselves were imagined as requiring dark and moist environments. Production Designer Herman Zimmerman said of the interiors, "We made three separate vacu-form [wall panel]s, brand new, for the Insectoids [...] even though we only saw environments for them twice – the inside of a control section for the ship, which was really a viewscreen shot ['Countdown'], and then the corridors and 'Hatchery'." (Star Trek: Communicator issue 151, p. 56) A typical corridor aboard one of these vessels was at first referred to, in the final draft script of "Hatchery", as "cramped". The next sentence stated, "The corridor has a modular design, comprised of repeating, interchangeable segments." The same teleplay specified that the Insectoid vessel was to have a "large" shuttlebay, and a "narrow" airlock (also referred to as a "vestibule") between two "large, circular" hatches, with "a small window" set into the center of one of those. In the final draft script of "Countdown", the control area was described as simply "a one-wall set."
In the Bethesda Softworks game Star Trek: Encounters, the Xindi-Insectoid ship was used for the "Xindi fighter", albeit rotated 180 degrees. It also appeared in the game Star Trek: Conquest, where it was identified as a Xindi scout.
In Star Trek Online the ship is known as the "Xindi-Insectoid Olaen-class Strike Wing Escort", and it is equipped with a hangar bay capable of launching Castroi-class fighters (whose design matches that of the Xindi-Insectoid scout ship) in battle. It also carries an Infectious Biomatter Generator device, which can be fired at enemies to degrade their ships' hull integrity and maneuverability.