This is actually a quite usefull thing, the documentation is available in the man pages of dpkg-query. In the list view of dpkg -l and dpkg-query the first three characters are relevant.
Desired action: u = Unknown i = Install h = Hold r = Remove p = Purge
Package status: n = Not-installed c = Config-files H = Half-installed U = Unpacked F = Half-configured W = Triggers-awaiting t = Triggers-pending i = Installed
Error flags: <empty> = (none) R = Reinst-required