Ubuntu – In the output of “aptitude update”, what do “Ign” and the other codes mean

I think it's referring to apt-get moo:

$ apt-get moo  
         (__)  
         (oo)  
   /------\/  
  / |    ||  
 *  /\---/\  
    ~~   ~~  
...."Have you mooed today?"...

To quote Wikipedia:

aptitude states that, unlike Advanced Packaging Tool, it "does not have Super Cow Powers". In apt-get "super cow powers" can be found by issuing the command apt-get moo. However, in aptitude issuing moo will give the user a prompt saying there are no easter eggs.

However, by issuing aptitude -v moo, then aptitude -vv moo, and so on the user will see a series of statements telling them to go away, some ending with a picture not unlike the original apt-get easter egg. Different versions of the program have different sequences.

And, as wRAR commented, the term “Super Cow Powers” is a direct quote from apt-get --help:

This APT has Super Cow Powers.

aptitude and apt-get work the same for many tasks, but for the most tricky cases, such as distribution upgrades (apt-get dist-upgrade vs. aptitude full-upgrade), they have different rules, and aptitude's rules are nearly always better in practice where they disagree.

The reason you see more documentation for apt-get over aptitude is mostly inertia: aptitude has not been the recommended front end to APT for all that long, so much of the existing documentation hasn't been updated, and there are plenty of people who recognise the advantages of aptitude over apt-get but use apt-get reflexively.

I've recently got to the point where I usually follow apt with a iand not a - when I type.

Postscript Note that the rules used in apt-get and aptitude are moving targets - as Hubert notes in comments, the upgrade path recommended from Debian Lenny now uses apt-get, not aptitude. This reflects the fact that apt-get keeps track of less state about the current package than aptitude, and so does not need to worry about APT state not being "clean", and because apt-get rules are smarter than they used to be. I still use and recommend aptitude over apt-get, but it is a more nuanced recommendation