Getting Information about packages in Ubuntu Linux

Finding packages:
How do you find the package that's got the feature you're looking for? First, do

sudo apt-get update

so your package list is up-to-date, and then try something like

sudo apt-cache search tunnel
sudo apt-cache search 'php.*'

That is how you tell apt to search the packages using REGEX (regular expression, a pattern-matching 'language') -- if your pattern uses any keystrokes that mean something to your command shell (e.g. [|?*] ) you'll need to quote them so that apt-cache will be able to see them, instead of having the shell expand the term to a list of file names that mean something else entirely.

NOTE -- apt-cache only knows about the package descriptions you've already downloaded.

Describing packages:
The simplest way to see the description or to get the checksum of a package:

sudo apt-cache show postgresql
sudo apt-cache show iproute

displays what the package is designed to do, version info and so forth.

Listing packages:
Which packages are installed? Do any packages need configuring?

sudo dpkg -l

Package contents:

sudo dpkg -L apache2
sudo dpkg -L apt

shows all the files -- with full path names -- that are provided by the package.

Finding which package contains:
How can you determine here did file XYZ come from? Here's how to find which package contains/supplies a certain file:

dpkg -S /etc/pam.conf