How to tell what has changed after an update?

[Patrick Doyle]

How can I tell what has changed after an update?  I presume (never
having looked at them very carefully) that the changelogs in
/usr/share/doc probably contain top level summaries.  Is there some
file I can look at or some apt/dpkg command I can run to determine
what packages were installed/updated when?  Presumably, if I sort that
by date, I can see what has changed after a recent update.

Suppose I wanted more details about a particular package, for example,
x11-common, (I don't know if that has changed recently or not, but
I've picked it because I happen to be trying to track down a
particular change in behavior of X11 right now).  That package is
listed as version 1:7.4+3ubuntu7.  I presume I could grab the
cvs/svn/git/mercurial source tree for version 7.4 of xorg, but how
could I tell what specific changes were enacted in going from 3ubuntu6
to 3ubuntu7?  How would I even know that version 1:7.4+3ubuntu6 was
the version I had installed prior to version 1:7.4+3ubuntu7?

Thanks for helping me figure this out.  As I said, I would rather
learn the proper tools to figure this out on my own, rather than just
finding out what the recent change to x11-common was (if there even
was a recent change).

--wpd