In the vmargin package, the \setmarginsrb command sets margins, among other things, with this syntax:
\setmarginsrb{left margin}{top margin}{right margin}{bottom margin}%
{headheight}{headsep}{footheight}{footskip}
The first four parameter set the width of the left margin, etc. What do parameters headheight, headsep, footheight, and footskip do? I gather that they have something to do with placement of headers and footers, but I'm having trouble finding documentation that explains this clearly. The vmargin documentation doesn't make it very clear, and I'd like to understand what I'm doing rather than experimenting until it looks right.
Feel free to send me to another question or an external a page that provides a good explanation.

vmargin: it's incompatible with packages such asateveryshithat's used byeso-picor others that try to place things in the background of a page. I think thatgeometryis much easier for setting the page parameters. – egreg Sep 08 '13 at 09:53