As simple as it is, there are only two kinds of bits but millions of variations. The two types are snaffle and leverage.
The snaffle has no leverage. The foot pounds you exert on the bit from your reins is 1:1 on the pressure exerted to the bit.
In the leverage bit there is a ratio of the purchase (the ring above the mouth piece that the headstall is attached to) and the shank (The piece below the mouth piece that the reins are attached to). There is also a chain or strap that goes under the chin, and using the mouth piece which acts like a fulcrum, applies pressure to the horse's jaw. The higher the ratio, the less pull you need on the reins to exert pressure. IE: if the ratio is 1:2 that means that for every 1 pound of pull you exert on the reins, then you are exerting 2x as much pressure to your horse's mouth, poll, and jaw.
The variations on these types are so numerous that there have been volumes written about each. How about narrowing it down a little? What discipline do you want to ride or at least English or Western?
Thanks for that link, I have also been wondering about different kinds of bits! I don't have time to read ALL that information right now but I saved it to 'my favorites' so I can go back and read it again and again
Is the ' D ' snaffle single jointed , double jointed , port mouthed , mullen mouthed , rubber mouthed , arch mouthed , sweet iron , happy mouth etc .- - The same would go for the pelham post some pics for a more detailed description.
Basically the D is a snaffle and the pelham is well a pelham ( pelhams should be properly used with two reins and a curb chain )