apt-get purge vs remove – Difference Between `apt-get purge` and `apt-get remove`

Not much. apt is a new command that supposed to merge several functions from apt-get and apt-cache into one command. It's still a little rough around the edges but here's the command listing from --help:

Basic commands: 
 list - list packages based on package names
 search - search in package descriptions
 show - show package details

 update - update list of available packages

 install - install packages
 remove  - remove packages

 upgrade - upgrade the system by installing/upgrading packages
 full-upgrade - upgrade the system by removing/installing/upgrading packages

 edit-sources - edit the source information file

The equivalent functions are designed to work in similar ways but it's not a proxy command (it's not calling the old ones - it's a new interface directly onto the Apt libraries) so there may be some edge-case changes.

There are also some obvious omissions (download, policy, etc) that power-users will miss and there are a whole raft of undocumented commands (purge still works but I can't find anything on it).

16.04 Update: A lot of the omissions have now been included but aren't yet documented, nor do they have Bash-completions. It's a shame it's taking this long to implement functionality that already exists in the codebase but oh well. My advice is that if you're used to an apt-{get,cache} command, try it on apt. It might work.

There's also a DIFFERENCES TO APT-GET(8) section in the man apt page that's interesting:

   The apt command is meant to be pleasant for end users and does
   not need to be backward compatible like apt-get(8). Therefore
   some options are different:

   ·   The option DPkgPM::Progress-Fancy is enabled.

   ·   The option APT::Color is enabled.

   ·   A new list command is available similar to dpkg --list.

   ·   The option upgrade has --with-new-pkgs enabled by default.

And if you want Bash-completions, I've had an attempt as writing a completions file for it already. These are included with later Ubuntu installs.

If you are familiar with MS-Windows, the usual mean of installing a software under that system is the setup.exe. As you've seen, Ubuntu uses packages.

The main (HUGE) difference is that packages have what is called dependencies so a system such as APT can go out and download all the necessary dependencies before installing the package you selected. Of course at times all dependencies were already satisfied, so just that one package gets installed.

A setup.exe and a package is an agglomeration of files within a single binary file. In most cases this is a compressed archive of some sort. Under MS-Windows, they generally use zip. Debian / Ubuntu packages use ar and tar (ar as a wrapper, tar to save all the files of the package.)

So... according to the general definition of software, one package can often be viewed as a software. The fact that it needs dependencies that automatically get installed is an artifact. However, some packages are not software per se. That is, a -doc package is probably just HTML or man files.

As a programmer, I would say that the software is the file that gets installed under /usr/bin (or an equivalent). These files are the actual brain of the software, the compiled program that is sent to the CPU in your computer to do stuff.