This symbol is used for:

the set of all integers.
the group of integers under addition.
the ring of integers.Extracted in Inkscape from the PDF generated with Latex using this code:

\documentclass{article}
\usepackage{amssymb}

\begin{document}
\begin{equation}
\mathbb{Z}
\end{equation}

\end{document}
Rendered by Latex from amssymb package's font
AMS