A metaphor is defined as "a figure of speech in which a word or phrase is applied to an object or action to which it is not literally applicable." This means that saying "She *IS* a typing machine" is a metaphor because it is simply not literally possible. The word "is" will be your hint that a sentence like this is a metaphor and not a simile. A simile usually has the keyword "like" in the sentence (For example: She was *like* a typing machine."
The main difference is that a metaphor claims that two things are the same while a simile claims two things are similar (you can remember this by the "simil" in both of these words!).
