Arrow Symbols in LaTeX

If $A \Rightarrow B$ and $B \Rightarrow C$, then $A \Rightarrow C$.

\[
  f: \mathbb{R} \rightarrow \mathbb{R}, \quad x \mapsto x^2
\]

\usepackage{amssymb}  % in preamble

$\overrightarrow{AB}$    % vector arrow over AB
$\overleftarrow{AB}$     % left arrow over AB
$\overleftrightarrow{f}$ % bidirectional over f

\usepackage{amsmath}

\[
  A \xrightarrow{\text{label}} B   % arrow with label above
  \quad
  A \xleftarrow{f}  B              % left arrow with label
\]