You've been given a lot of good tips and information, but it seems to me that you've got it piecemeal. So:
In KDE there are several ways of setting up programs to start automatically at login.
First you can specify whether a new login should start with an empty session, restore your previous session (i.e. restart the programs running at you last logout), or restore a manually saved session. ('Systemsettings', a.k.a. 'Configure Your Desktop'-->'System Administration'-->'Startup and Shutdown'-->'Session Management'.)
But even if you've ticked 'Start with an empty session', any executables found in a number of autostart directories will be run at login. Some of these directories are user specific; some of them are global, i.e. the programs in them are started at login regardless of who is logging in.
User specific autostart folders:
Global autostart folders:
You can actually set up KDE to use any folder as its default autostart folder. ('Systemsettings'-->'Account Details'-->'Autostart Path:') Typically the default folder would be /home/<yourname>/.kde4/Autostart, but it doesn't even have to be named 'autostart' or 'Autostart'.
And even if you've picked ~/.kde4/Autostart as your default autostart folder some apps may in different ways end up in ~/.config/autostart instead. It depends on how the link or .desktop file is created.
Often the files in the autostart directories will be .desktop files (from which programs can be started with lots of options), but they may as well be shell scripts or symbolic links to executables.
When you add a program from the GUI ('Systemsettings'-->'Startup and Sutdown'-->'Autostart') a desktop file will normally be created in your default autostart folder, but you can change that.