From Halopedia, the Halo wiki
The needler is often seen with an upper cowling where the crystal ammunition exposed sticks out of. The stock of the weapon is often at the bottom of the weapon and doubles as the weapons power supply. The channel brace of the weapon is where the crystal ammunition is fired out of. When preparing to fire and launch the crystals, the needler breaks them, and charges their chemical mixture.
The needler makes use of Subanese crystal designed to home in onto targets as its ammunition. When in the weapon the crystal is exposed above the weapon. Different variants of the weapon using calibrated shard of the crystal.
After the crystal shards have been charged and launched from needler, the shards home towards a target if one is present. If a shard hits a target it becomes chemically reactive and usually detonates, where if more than one shard is in a target it explodes in a "supercombine" chain reaction with the other shards.
Prior to 2552, the needler was manufactured by the Sacred Promissory on High Charity. After the fall of High Charity during the Great Schism, the manufacturing of the needler fell to Lodam Armory.
The needler was a weapon favored by poorly trained Covenant soldiers due to its "supercombine" explosion. Used throughout the Human-Covenant War, humanity recorded some needlers in 2533 in their Type classification system as the Type-33 needler. 
The Needler has appeared in several games in the Halo franchise.
List of appearances