Most of the literature refer to "cobalamin" or "cyanocobalamin." Other forms of "cobalamin" (B12) have been developed since cyanocobalamin, but unfortunately most literature is written as if cyanocobalamin exclusively defines B12. Cyanocobalamin is a synthetic version of the vitamin. It works for most people, but many people cannot convert it efficiently to a form that can be used, and unconverted it is useless.
Hydroxocobalamin is preferred over cyanocobalamin by those who are familiar with it. It is useful in many cases more than cyanocobalamin, and it has better lasting power.
Methylcobalamin has some advantages neither of the other types do. It is the natural form of B12, made by the bacteria that must form natural B12. It does not need to be converted, and it has been proven very helpful not only for those who can be shown deficient in B12 but in many who with correct testing cannot: including some ALS and MS cases.
Methylcobalmin is easier to find that years ago, but many health food stores and pharmacies still have no clue. Often it is easier and less expensive to choose an excellent brand name and purchase it online, in at least 1000 mcg (1 gm) doses. Do not fall for marketing that requires you to spend a lot of extra money on something of equal or lesser value. Avoid hype.
There are alternate spellings of "methylcobalamin" in countries other than the USA.
More to come...