lamer question display installed pkgs

[reader at newsguy.com]

Its been a good while since I used apt-get and apt-cache.  I once, yrs
ago ran debian for a while.

I can't remember how to get a list of installed pkgs.  
After browsing both man apt-get and apt-cache... I still didn't see a
switch that shows installed pkgs.

There is always `ls /var/cache/apt/archives|less'  but I thought I once
knew an apt [something] command to show installed pkgs.

It appeared in a kind of roster with other pkg information too, as I
recall but it has been years ago so may have been something besides
the apt tools.