それとも is a conjunction in Japanese that may be translated as ‘or’, or ‘otherwise’. It is used to highlight alternative options or choices between (A) and (B). それとも is a combination of それ, and the particles と, and も. This grammar structure will only be used when the options are limited to either (A), or (B).
それとも will appear either after a comma, or at the beginning of new sentences.

• 今日（きょう）は文法（ぶんぽう）の勉強（べんきょう）をしますか。それとも漢字（かんじ）の勉強（べんきょう）をしますか。
  Do you want to study grammar today? Or rather, do you want to study Kanji?
• 来週（らいしゅう）はどこに行（い）きたい？動物園（どうぶつえん）に行（い）く？それとも家（いえ）でゆっくりする？
  Where do you want to go next week? Do you want to go to the zoo? Or rather do you want to relax at home?
• 今日（きょう）は海（うみ）に行（い）こうか、それとも川（かわ）に行（い）こうか、迷（まよ）うな。
  I don't know whether to go to the ocean or the river today.

In many cases, (A) and (B) will be opposites of each other. However, this is not a requirement for using それとも.