Ubuntu – the difference between xtightvncviewer and vncviewer

vncviewer is a virtual package that is only provided by xtightvncviewer.

They're (currently) the same thing.

To double-check, let's look at the files themselves:

oliver@ubuntuDesktop:~$ ls -lha /usr/bin/vncviewer 
lrwxrwxrwx 1 root root 27 2012-04-07 14:35 /usr/bin/vncviewer -> /etc/alternatives/vncviewer
oliver@ubuntuDesktop:~$ ls -lha /etc/alternatives/vncviewer
lrwxrwxrwx 1 root root 24 2012-04-07 14:35 /etc/alternatives/vncviewer -> /usr/bin/xtightvncviewer

So, vncviewer is just a placeholder in the system for whatever package you install that provides a vncviewer. In this case there is only one option xtightvncviewer.

There are several virtual packages. They are defined by the Provides: setting defined in other packages. The best list of virtual packages I found is this one: http://www.debian.org/doc/packaging-manuals/virtual-package-names-list.txt

Virtual packages are very useful for package maintainers. You can create your own package and say "If you want to use this package, you'll need another package that provides an ftp-server." The package doesn't care which one, as long as you installed a package that provides an ftp-server.