dpkg is a tool to build, install, remove and manage packages in Debian. It is called by apt-get to actually install the packages after apt-get has resolved dependencies and downloaded packages.

dpkg is a useful tool to find out information about individual packages:

to see which packages are installed: dpkg -l
to look at a package description: dpkg -I PackageFile.deb
to look at package contents: dpkg -L Package
to search for a filename within installed packages: dpkg -S SearchTerm (to also search in not installed packages you can use apt-file).
to (download and) install packages:dpkg -i *.deb
to find what package provides the installed file /bin/ls: dpkg --search /bin/ls
to find what program provides the file Xlib.h:auto-apt search Xlib.h

Dpkg

See also