For most of our examples, we don’t use a program called latex but instead
one called pdflatex. This is one of a family of related programs, all
of which are ‘descendants’ of latex. We have chosen pdflatex because
it is perhaps the most widely-used system and directly produces PDF files.
As described earlier, LaTeX is built on a system called TeX. We
call LaTeX a ‘format’: a collection of macros (instructions and commands) that
TeX understands. When you run pdflatex, you are actually starting a
program called ‘pdfTeX’ with a pre-loaded ‘LaTeX format’. We normally call
pdfTeX an engine: a program that understands TeX instructions.
There are three engines in common use today:
We will cover XeTeX and LuaTeX later: the main thing we need to know now is that they are capable of loading operating system fonts, whereas pdfTeX can’t do that.
If you are in Japan, or writing lots of Japanese, you might also come across pTeX and upTeX. These are specialised engines for vertical typesetting. LuaTeX can also do a lot of this, but at the moment upTeX, in particular, is still the most popular system for Japanese.