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.

First character:

Desired action:
u = Unknown
i = Install
h = Hold
r = Remove
p = Purge

Second character:

Package status:
n = Not-installed
c = Config-files
H = Half-installed
U = Unpacked
F = Half-configured
W = Triggers-awaiting
t = Triggers-pending
i = Installed

Third character:

Error flags:
<empty> = (none)
R = Reinst-required

Enjoy!