Variants are differences between two copies of a text. They’re most visible in manuscripts, where no two copies are quite the same, whether through accident or intention: a scribe may misread the copy from which he works, or try to make sense of a passage by altering it. But although the number of variants is sharply reduced by printing, they’re still plentiful.
Twentieth-century textual critics distinguish two broad classes of variants, substantive and accidental. Substantive variants are those that change the sense of the text: the substitution of one word for another, for instance. Accidental variants are those that don’t affect the meaning: the use of uppercase or lowercase letters, for instance; changes from British to American spelling; or differences in line-end hyphenation. Of course, determining whether any particular variant is substantive or accidental is often a judgment call.
Note: This guide is still in the early stages of development.
Three question marks mean I have to write more on the subject. Bear with me.