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.)
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.
2 <endline><endline>
Two consecutive ends of line finish a paragraph. (One end of line behaves like a space.)
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!
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.