The main purpose of the preview package is to extract environments as images. To use it, the following line must be added to the preamble of the document.
\usepackage[active, tightpage]{preview}
where options active and tightpage are specified to activate the package, and to produce separate dimensions for every page, respectively.
The macro \PreviewEnvironment sets which environments must produce preview images.
\PreviewEnvironment{tikzpicture}
\PreviewEnvironment{tabular}
The code snippet above, for instance, makes all tikzpicture and tabular environments in the document produce images.
Full example:
\documentclass{article}
\usepackage{tikz}
\usepackage[active, tightpage]{preview}
\PreviewEnvironment{tikzpicture}
\PreviewEnvironment{tabular}
\begin{document}
\begin{figure}
\begin{tikzpicture}
\draw[step=1cm,gray,very thin] (-1,-1) grid (4,4);
\draw[thick,->] (0,0) -- (3.5,0);
\draw[thick,->] (0,0) -- (0,3.5);
\foreach \x in {0,1,2,3}
\draw (\x cm,1pt) -- (\x cm,-1pt) node[anchor=north] {$\x$};
\foreach \y in {0,1,2,3}
\draw (1pt,\y cm) -- (-1pt,\y cm) node[anchor=east] {$\y$};
\end{tikzpicture}
\end{figure}
\begin{center}
\begin{tabular}{ |c|c|c| }
\hline
cell1 & cell2 & cell3 \\
cell4 & cell5 & cell6 \\
cell7 & cell8 & cell9 \\
\hline
\end{tabular}
\end{center}
\end{document}