What Text Are the Translators Translating?

What is it that Bible translators translate when they are translating? Let me focus on the New Testament, my main area of expertise. When a translator wants to make an English version of, say, Mark (what I say about Mark will be true of all the books of the NT), what does she actually translate into English?

Obviously she cannot take Mark’s original manuscript and translate it, since we don’t have it. Or the first copy of the original, or a copy of the copy of the original. We have hundreds o...