I've just downloaded both packages to inspect them, the process for comparing the packages can be found on the bottom.
It contains the Flash player library and configures itself as default for Internet browsers. It was only available for 32-bit systems, but since October 2011 a 64-bit version is available too for Ubuntu Lucid and later. This can be installed from the partner repository, see also How do I enable the "partner" repository?
It extracts the 32-bit Flash player library from the
adobe-flashplugin package (which is retrieved from http://archive.canonical.com/) and configures itself as default for Internet browsers. Additionally, it installs a 32-bit compatibility layer for 64-bit systems.
There is no change in the installed Flash player for 32-bit systems. For 64-bit systems,
flashplugin-installer installs the 32-bit version of Flash and a related compatibility library which may be slower and less stable. I would recommend using the native 64-bit library instead (
adobe-flashplugin was found in http://archive.canonical.com/pool/partner/a/adobe-flashplugin/?C=M;O=D (I haven't enabled the partner repository):
flashplugin-installer can be downloaded using
apt-get download flashplugin-installer. For comparison, I downloaded the 32-bit package from http://packages.ubuntu.com/natty/i386/flashplugin-installer/download.
dpkg-deb --control package_version.deb target-dir - extracts the control files (e.g. post-installation scripts) to directory
dpkg -x package_version.deb target-dir - extracts the contents of the package to
dpkg --contents package_version.deb - shows the contents of the package
diff -Nur one two - compare directories
postinst scripts were the most interesting as it makes the only difference between the packages.