[exmadscientist]

At least in the fields I've published in, first author does the work (or at least most of it, almost always including writing the paper) and last author secured the funding. Sometimes authors are listed in alphabetical order, in which case one or two are usually listed as "corresponding author". The less-senior corresponding author usually did the work, and the more-senior corresponding author is usually either the one who secured the funding, or a long-term member of the research group who is likely to actually be around to answer correspondence in the future (and who also probably helped out enough to be worth corresponding with).