Bungie mentions a Plasma Repeater, Plasma Launcher and a Focus Rifle here where it lists the weapons.
Focus rifle is just the covy sniper. Plasma Launcher looks extremely epic and I have no clue about the Repeater.
The plasma repeater was the "revamped plasma rifle." It is the one that fire more slowly the longer you stay on the trigger, but more accurate. To cool it, you reload. They still have plasma rifles because plasma repeaters are too good to be in campaign. So the needle rifle is the covenant answer to the DMR and the plasma repeater is the covi assault rifle.
From what I've gathered from extensive reading, The Focus Rifle takes the PLACE of the Beam Rifle, but it is actually much more related to the Hammer of Dawn in Gears of War regarding use and effect.
Basically, from my understanding, the Repeater is more like the human Assault Rifle than the Plasma Rifle ever was. The Launcher is the primary Covenant heavy weapon, and is the equivalent to the Rocket Launcher or Missile Pod. It was also the "mystery sound" that Bungie posted in one of their weekly updates. The Focus Rifle is very much like a combination of the Beam Rifle and the Sentinel Beam, NOT the Spartan Laser.