layers in tikz in latex

To define a layer in tikz you would first in the preamble define a new layer with a defined name (here i chose "layer") and then set the order of the layers especially in comparison to the standard "main" layer.

\pgfdeclarelayer{layer}    % declare layer
\pgfsetlayers{layer,main}  % set the order of the layers (main is the standard layer)

in the plotting step you can then explicitly plot on a different layer by using the pgfonlayer environment

\begin{pgfonlayer}{layer}
      \draw[ultra thick] (0.0,0.0) -- ++ (1.0,0.0);
\end{pgfonlayer}