Questions regarding the distinction between XeTeX and XeLaTeX and how they relate to TeX and LaTeX?

tex

tex is an executable file--a runnable program. By default it has the Plain TeX format preloaded, so when you run tex file.tex it is prepared to read macros defined in plain.tex. It produces DVI output.

pdftex

pdftex is another version of the tex executable that includes certain extensions to the original program (the e-tex extensions, including additional memory registers). By default this produces PDF output directly instead of DVI. It also has the Plain TeX format preloaded.

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latex

The command latex calls the pdftex program (or "engine") but with the LaTeX format preloaded instead of Plain TeX, and set to produce DVI output.

pdflatex

The command pdflatex calls the pdftex engine but with the LaTeX format preloaded instead of Plain TeX, and set to produce PDF output.

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xetex

xetex is a different executable file from tex and pdftex. It includes extensions to the original program to allow Unicode input and right-to-left typesetting, among other features. By default it has the Plain TeX format preloaded and produces PDF output.

xelatex

The command xelatex calls the xetex engine but with the LaTeX format preloaded instead of Plain TeX and produces PDF output.

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luatex

luatex is yet another distinct executable file, a partial reimplementation of tex that incorporates the lua scripting language. Like xetex, luatex is built to accept Unicode input. It has the Plain TeX format preloaded and produces PDF output.

lualatex

The command lualatex calls the luatex engine with the LaTeX format preloaded instead of Plain TeX, and produces PDF output.

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Examples

Plain TeX format

You can successfully compile this Plain Tex file with either tex, pdftex, xetex, or luatex:

Test of Plain \TeX.\bye

Plain TeX format with Unicode input

If you use Unicode characters, you will only be able to print those characters by running xetex or luatex and selecting a font that includes them:

\font\tenrm=ecrm1000 \rm
Test of Unicode with Plain \TeX: Matthäuspassion, esdrújulo.\bye

LaTeX format

To compile a LaTeX document, use latex, pdflatex, xelatex, or lualatex:

\documentclass{article}
\begin{document}
Test of \LaTeX.
\end{document}

LaTeX format with Unicode input

If you use Unicode characters, you can print them in two ways:

1. use pdflatex and set the proper input and font encoding using inputenc and fontenc packages, or
2. use xelatex or lualatex and select a Unicode font using the fontspec package.

For pdflatex:

\documentclass{article}
\usepackage[utf8]{inputenc}
\usepackage[T1]{fontenc}
\begin{document}
Test of Unicode with \LaTeX: Matthäuspassion, esdrújulo.
\end{document}

For xelatex or lualatex:

 \documentclass{article}
 \usepackage{fontspec}
 \setmainfont{Latin Modern Roman}
 \begin{document}
 Test of Unicode with \LaTeX: Matthäuspassion, esdrújulo.
 \end{document}