begin{tikzpicture}[%
  stuff/.style={%
    draw,
    text=Bla bla
  },
]

node at (0,0) [stuff] {};
node at (0,1) [stuff] {};
node at (0,2) [stuff] {};
node at (0,3) [stuff] {};
end{tikzpicture}

documentclass{article}
usepackage{tikz}
begin{document}
 begin{tikzpicture}[%
  stuff/.style={%
    draw,
    font={A}}
]

node at (0,0) [stuff] {};
node at (0,1) [stuff] {};
node at (0,2) [stuff] {};
node at (0,3) [stuff] {};
end{tikzpicture}

end{document}

documentclass{article}
usepackage{tikz}
begin{document}
 begin{tikzpicture}[%
  stuff/.style={%
    draw,
    node contents={A}}
]

node at (0,0) [stuff];
node at (0,1) [stuff];
node at (0,2) [stuff];
node at (0,3) [stuff, node contents={B}];
end{tikzpicture}

end{document}