\documentclass{article}

\newcommand{\IFF}{if and only if}

\begin{document}
This is my \ emph {first} document prepared
in \ LaTeX
\ LaTeXis fun
\LaTeX{} is fun.  \\ 
\LaTeX\  is fun


A function $f:X\to Y$ is continuous \IFF{} $f^{-1}(U)$ is
open in $X$,
for every open set $u\subset Y$.


\end{document}