Keith probably knows the specific numbers bettre than I do, but you're right, since it's my favorite I do know.
It's a modular ship, cap of 2 per mk. Each one gets mk modules. There's five modules: a FF generator (that I never use), and four counter modules: Shell/Laser/Missile/Energy (which is a broad definition including negative energy, energy waves, implosion shots, impulse reactive shots, flame waves).
The way it works is that when an incoming shot of the appropriate type enters its area of effect, the Protector fires a counter shot that nullifies the shot in question. That takes one shot from the Protector. If the shot it's countering is over 1000 (I think) damage, then it takes more shots (a 4500 damage shot would take 5 Protector shots to counter).
The Protector modules regain 10 shots/second and have a cap of 100 shots available. So they can stop a big alpha strike salvo, but then have an effective shots per second and DPS limit on what they can stop. If you outfit them for a specific damage type, you can stop a LOT of that type, or you can use them more well rounded by having a wide variety of modules. They can be overwhelmed either by huge damage shots, or by swarms that fire so many shots they simply can't stop them all. Amusingly, Laser Gatlings are a PITA for them because they are a big swarm and rapid fire small shots, which is the worst thing for Protectors to try and stop.
A non-modular version could work where it simply stops any eligible type of shot, up to its limit. That'd make them easier to use and not let you abuse them to counter a SuperFortress, but because they have a finite amount of DPS and shots they can react to, a strong defense still beats them. That's why I like to call them "counter DPS" ships: their attack is effectively anti-damage.
It's a very unique mechanic that nothing else in the game has, IIRC.