"tedious" is used when referring to something that requires you to spend a lot of attention and effort on minor details. So it is used for an action that you do, for example : tedious work . "monotonous" is something that is always the same : monotonous life , monotonous view, monotonous voice (someone who is spelling all words the same without accents and without emphasizing certain words). Usually "monotonous" implies that something is boring.

Actually, those two words are synonyms of each other. :) They both mean essentially the same thing: slow, boring, repetitive, or lacking variety and interest.