In order to use my university's printing shop, they said they need a PDF x-1a format. I noticed in Pages that I can print as "Save as PDF-X" but a little googling says this is not exactly the PDF x1a format, but another one. Googling did not turn up exactly how to get the x-1a format.
If they can take PDF-X1/a, then they can take any of the others as well; you shouldn't have any problems.
PDF-X1/a was simply the first in a long line of iterations of a new PDF schema that basically does a lot of preflighting for you (it's one of the best things to happen in the printing industry in a long time). It means your document is "press ready", which mainly applies to SWOP presses but can apply to small printers as well.
As a footnote to all of that... You can also output press-ready PDFs using the Mac OS X-supplied "Save as PDF" button in the print dialog box from just about any application.