As far as I understand it (a few days of research here and there), there are two major TeX engines: pdfTeX and XeTeX. pdfTeX is the "standard", having been around since the early 1990s, renders straight to PDF, and improves on some minor formatting issues with original TeX.
XeTeX, on the other hand, also outputs PDF, can use any system font without complication, and can accept Unicode input by default. And yet for some reason it's not the default engine in any of the TeX distributions.
Do I have this right? Why is pdfTeX still the standard? Which do you use?