\documentclass[varwidth=true, border=2pt]{standalone}
\usepackage{tikz}
\usetikzlibrary{arrows,positioning} 
\tikzset{
    %Define standard arrow tip
    >=stealth',
    % Define arrow style
    pil/.style={->,thick}
}

\begin{document}
  \begin{tikzpicture}
      \node (a)[vertex] at (0,0) {};
      \node (b)[vertex] at (0,1) {};
      \node (c)[vertex] at (1,0) {};
      \node (d)[vertex] at (0,1) {};

      \foreach \from/\to in {a/b,b/c,c/d,d/a,a/c,b/d}
        \draw[line width=2pt] (\from) -- (\to);
  \end{tikzpicture}
\end{document}
