TeX
TeX was the first language to get its own StackExchange site.
(Note that we will show examples in plain TeX mostly; if any example is to be compiled using LaTeX, it will be clearly stated.)
8 \def\x{}
Simple definition. This defines a macro ("control sequence" in proper words) which does not take an argument and expands to ... well ... nothing :) (Do not be confused: "nothing" is not "relax", they substantially different.)
7 $$a+b$$ or \[a+b\]
Two snippets that somehow produce the same output, it's a displayed equation showing simple
a + b
(just centred of course). The first is the correct thing to do in plain TeX. If only people knew how wrong it is in LaTeX, where the second one should be used!
6 \relax
TeX is one of the languages that have its own "do nothing" action. However, \relax is more than just that, and this is related to the fact that it is an expansion language: All macros are expanded, and in some contexts, executed. The power of \relax is that it is executed to nothing, but it cannot be expanded, i.e., it survives any expansion unmodified. This in turn is one of the strenghts (and threats at the same time, especially for newcomers) of TeX.
5 X\bye
A minimal document that compiles producing an output; we get a page with a 10-point "X" at the top and a centred page number "1" at the bottom. ... "X," said Tom, and added, leaving: "Bye!"
4 \bye
When you say "bye" to TeX, it finishes the document; namely it closes the current paragraph if any is open, ships out the last page, and ends. Without this command, no document can be successfully produced.
3 $a$
Prints the mathematical symbol "a", that can stand for instance for a variable, function or a constant. Without the dollars, it would be the text symbol "a"; it is necessary to distinguish these two!
2 <endline><endline>
Two consecutive ends of line finish a paragraph. (One end of line behaves like a space.)
1 %
The percent sign starts a comment; the comment runs until the end of line, and eats the end of line together with leading whitespace on the next line.