By default Debian still uses Bash for interactive user shells, but since Squeeze (early 2011), /bin/sh is provided by Dash.
Dash is used because it's faster than Bash and has less dependencies so its more resilient to failures.
for more info see https://wiki.debian.org/Shell, https://wiki.debian.org/DashAsBinSh and https://packages.debian.org/wheezy/dash