First author = did most of the work, usually the one who wrote the paper
Last author = usually the PI whose grant paid for the work
Everyone in-between = other people who worked on the paper, often in descending order of contribution
Sometimes the first couple of authors will have a star by their names with a footnote that says "these authors contributed equally to this work." In those cases, it's advantageous to have a last name that starts with a low letter of the alphabet, because most people don't notice the footnote, and it doesn't show up in the bibliographic listing of the article.
Sell crazy someplace else, we're all stocked up here.