.deb files are regular ar archives. You can manipulate the file by using ar command.
For example, I have a package called lynx_2.8.8dev.15-2_all.deb in a centos box.
$ cat /etc/issue CentOS release 6.4 (Final) Kernel \r on an \m $ ls -lh
total 4.0K
-rw-rw-r--. 1 foo foo 3.9K Jan 18 2013 lynx_2.8.8dev.15-2_all.deb
To extract the file, just run ar x <.deb.filename>
$ ar x lynx_2.8.8dev.15-2_all.deb
You will see 3 files being extracted, control.tar.gz, data.tar.gz and debian binary.$ ls control.tar.gz data.tar.gz debian-binary lynx_2.8.8dev.15-2_all.deb
debian-binary is a text file containing deb packaging version number.
$ cat debian-binary 2.0
control.tar.gz contained control, preinst and md5sums.
$ tar -tf control.tar.gz ./ ./md5sums ./control ./preinst
data.tar.gz is a compressed file containing the files which are going to be installed on the system.
$ tar -tf data.tar.gz ./ ./usr/ ./usr/share/ ./usr/share/doc/ ./usr/share/doc/lynx/ ./usr/share/doc/lynx/copyright ./usr/share/doc/lynx/changelog.Debian.gz
