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"):
Most have (B) and (W) markings on them, meaning Black and White respectively. A and A II are interchangeable.
Most Canon rings have a hinge that allows the collar to be removed without dismounting the lens. Only Canon "B" rings do not have a hinge and do require the lens to be dismounted to remove the ring. 3rd party rings come in both hinged and hinge-less designs. Some users prefer the hinge-less version for ease of use when using the lens collar to transition from landscape to portrait orientation. Other users prefer the ability provided by the hinge to add/remove the collar without dismounting the lens.
Lenses that use the Tripod Mount Ring B (W) have lugs that fit inside a grove in the lens collar. The collar also has slots to allow clearance for the lugs when the collar is removed.
*The Tripod Mount Ring B (B) pictured at Canon's online store appears to have a hinge. This is in fact an incorrect picture of a 3rd party Tripod Mount Ring. (you can read "for Canon" on the mount) The Tripod Mount Ring B (B) pictured at B&H is clearly marked correctly and is not hinged and does include the slots and groove for the lugs on the lenses that include the Tripod Mount Ring B (W) as standard equipment. The only lens that uses the Tripod Mount Ring B (B) not supplied with the collar as standard equipment is the EF 100mm f/2.8 Macro (non-L) which does not have lugs that fit in the slots and groove. There is a plastic adapter included with the B(B) ring that fits the EF100mm 2.8 USM Macro lens for use with the B(B) ring.
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.