The differences between Helvetica and Arial are very subtle, though it is imperative we learn to discriminate between the two because a) One of them is evil; b) you never know when you are going to be stuck in a situation where your life will depend on your knowing which of the two has a spur on the capital G. Get learned. After you get learned, go here and take the quiz.
I know I have posted this before, but it is so fun! Kill Arial.