\documentclass[a4]{article}
\usepackage{tikz}

\begin{document}

Draw a box and label the corners and the center:

\begin{tikzpicture}
	\draw (0,0) -- (0,1) -- (1,1) -- (1,0) -- (0,0);
	\node [below left] at (0,0) {$a$};
	\node [above left] at (0,1) {$b$};
	\node [above right] at (1,1) {$c$};
	\node [below right] at (1,0) {$d$};
	\draw [fill] (0.5,0.5) circle [radius=0.5pt];
	\node [above right] at (0.45,0.45) {\small {$e$}};
\end{tikzpicture}

\bigskip \hrule \bigskip

Draw some lines:

\bigskip

\begin{tikzpicture}
	\draw [|-] (0,0) -- (2,0);
	\draw [|-<] (0,0.5) -- (2,0.5);
	\draw [|->] (0,1) -- (2,1);
\end{tikzpicture}

\bigskip \hrule \bigskip

\paragraph{An experiment with polar coordinates}

This does not work: 

\vspace{1.5cm}

\begin{tikzpicture}
	\tikz \draw [->] (0,0) -- (0:1);
	\tikz \draw [->] (0,0) -- (72:1);
	\tikz \draw [->] (0,0) -- (144:1);
	\tikz \draw [->] (0,0) -- (216:1);
	\tikz \draw [->] (0,0) -- (288:1);
\end{tikzpicture}

\bigskip

But this does:

\bigskip

\begin{tikzpicture}
	\foreach \a in {0, 72,...,288}
		\draw [->] (0,0) -- (\a:1);
\end{tikzpicture}

\end{document}