documentclass{article}

usepackage{tikz}

begin{document}
  begin{tikzpicture}%
    draw[help lines] (0,0) grid (1,1);

    node [draw, circle] at (0,0) {};
    node [draw, circle] at (1,1) {};

    draw[->] (0,0) -- (1,1);
  end{tikzpicture}
end{document}

documentclass{article}
usepackage{tikz}
begin{document}
  begin{tikzpicture}%
    draw[help lines] (0,0) grid (1,1);
    node [draw, circle]          (c1) at (0,0) {};% special syntax
    node [draw, circle, name=c2]      at (1,1) {};% name key
    draw[->] (c1) -- (c2);
  end{tikzpicture}
end{document}