What are the types of Canon's tripod mount rings? I've heard about A, B, C and D types, but have no idea what it means or what the difference between them is.
The different types are:
The interior ring dimension for each are (note well they do not go "in order"):
They have (B) and (W) markings on them, meaning Black and White respectively. A and A-2 (II) are said to be interchangeable.
According to different sources, B also fits for 100-400mm f4.5-5.6 and both IS and non-IS 70-200 f2.8.
I've never heard of A B C D etc., but a quick search on the net would suggest these are types of ring mounts for lenses, probably for long lenses in which you would not mount your camera but the lens itself. I imagine the letter designation has something to do with the amount of weight the ring can handle.
If you're looking for pods and accessories, you've got to check out http://www.gitzo.com/. These pods are amazing. Pricey yes, but you'll buy one and be set. The hardware is SO much stronger than a Bogen, virtually indestructible.
As for heads and mounts, my favorite I've found is from a little company in San Luis Obisbo, CA. called Really Right Stuff. Their mounting gear is AWESOME. I use their ball mount on my Gitzo and their pano gear as well. Super strong, spirit levels on everything, and totally customizable.
Also, check this discussion out...you may find it useful. http://forums.steves-digicams.com/tripods-heads/152976-tripod-abcd.html
What equipment are you trying to mount?
I haven't really researched this, but I think that you either get a tripod mount ring with your lens (I got one with Canon 70-200 telezoom), or you should mount the camera.