There are several threads and stickys about bits. Can you do a search on them?
Therea are basically two kinds of bit; snaffles and leverage bit
Snaffles include: Mullen mouth types (no break in the middle), single jointed (very typical English bit), double (french link, Dr. Bristol, KK, etc.)
What these all have in common is that they have a direct connection from the rider's arm to the bit and the amount of pressure exerted by the rider is transmited at a 1:1 ratio. Kinf of a "what you pull is what he gets"
Leverage bits (Pelhams, kimberwickes, curbs , tom thumbs, double bridles, etc.)
These use the concept of the lever to increase the force transmitted by the rider's pull to the bit. So, there is always an offset from the contact point of the rein to the bit which creates leverage, and the greater the distance of the contact of rein to bit and where the bit is connected to the mouthpiece side, the greater the ratio of pressure will be. So, a curb with long shanks has a greater ratio of pressure than a Kimberwick. Of course, as I learned recently on this forum, the angle of the shanks also will affect how quickly the contact is felt as pressure.
Leveraged bits usually use a curb chain also, which transmit pressure to the lower jaw as the reins are pulled and the lever action begins.
I hope I explained things ok. I know that some other folks did a fantastic job of it on several posts. Good luck on your test!