Q:

How do we know what applications are installed in Linux?

I'm trying to understand the following in Linux.  In windows we have the services that run or "startup". When we install an application it could be installed as a service so that it starts automatically.   
But if an application is not installed as a service we usually can see it in the Start -> Programs menu. So we know what applications are installed.  
In Linux what is the equivalent?  I understand that the equivalent services are in /etc/init where the services start/stop.  But I assume that if I install a package it does not necessarily create a startup script in /etc/init right?   
So how does one know what has been installed and is available in Linux (like we can in Windows from Start -> Programs)?  
Note: I'm asking about CLI mode. I guess in a desktop version one can see the relevant icons in the various menus (e.g in Kubuntu from Application -> Internet -> Firefox).

A:

Many questions. Let's take a couple and see if we can't clear things up.

Q1
I understand that the equivalent services are in /etc/init where the services start/stop. But I assume that if I install a package it does not necessarily create a startup script in /etc/init right?

No when you install applications on Linux distros (ones that make use of package managers such as dpkg/APT, RPM/YUM, pacman, etc.), as part of the software being installed the package manager has a scripting "feature" similar to those found in Windows that can add scripts, create scripts, add users to the system, and start services after they're installed.

Q2
So how does one know what has been installed and is available in Linux (like we can in Windows from Start -> Programs)?

Easy. The same package managers that I mentioned above have commands you can use to query the system to find out what applications have been installed, what files are related to these packages etc. etc.
Example
On Red Hat based distros you can use  the command rpm to find out information about the packages installed.
$ rpm -aq | head -5
libgssglue-0.4-2.fc19.x86_64
pygame-1.9.1-13.fc19.x86_64
perl-HTML-Parser-3.71-1.fc19.x86_64
ibus-libs-1.5.4-2.fc19.x86_64
libnl-1.1-17.fc19.x86_64

To find out what files are part of a package:
$ rpm -ql pygame | head -5
/usr/lib64/python2.7/site-packages/pygame
/usr/lib64/python2.7/site-packages/pygame-1.9.1release-py2.7.egg-info
/usr/lib64/python2.7/site-packages/pygame/LGPL
/usr/lib64/python2.7/site-packages/pygame/__init__.py
/usr/lib64/python2.7/site-packages/pygame/__init__.pyc

How can it show me just the executable pieces to that are included in the package (the applications)? Most of the time executables are installed in certain locations on Linux, /usr/bin or /bin are 2 such directories. I usually search the RPM packages like so for these:
$ rpm -ql pygtk2 | grep "/bin"
/usr/bin/pygtk-demo

$ rpm -ql httpd | grep -E "bin/|sbin/" | head -10
/usr/sbin/apachectl
/usr/sbin/fcgistarter
/usr/sbin/htcacheclean
/usr/sbin/httpd
/usr/sbin/rotatelogs
/usr/sbin/suexec

